This question is posed with two assumptions:

a) the boarder is on a (mostly) open piece of land (no natural or man-made walls cutting off travel)

b) each country has a reason to protect the boarder and stop people from passing freely. Not that the countries are necessarily hostile, but at least cautious. 

When your party cross a boarder between countries, what sort of resistance do they meet? I know alot of the finer details come down to the specific relationship the two countries hold as well as the internal politics of those countries, but I am looking for just some general guidelines. 

If they are really trying to stop people crossing, I'd imagine they'd have regular patrols along the borders, and potentially some fortifications. Depending on how long this situation has been going on and how wealthy the countries are, fortifications could range from a bunch of sacks of sand piled up to watchtowers to a proper fortress -- maybe some combination of a central fortress with watchtowers further out. There would probably be a couple points (at least one), likely along major roads or waterways, where people are permitted to cross after a very through inspection, if only to allow some trade.

Unless the party members (who we can safely assume are heavily armed and posses obvious magical capabilities) were very well known, if they attempted to cross at someplace that wasn't one of these checkpoints, they'd likely be arrested, and tried as spies for the other country. The trial might take a while, but they'd probably not hold back, like bringing in someone with a zone of truth spell to really get to the bottom of it. Then, even if they were found innocent, there would still be a pretty steep fine (at least) for violating the border and not crossing at a checkpoint, then probably they'd be deported back in to the first country.

A border patrol with a well know pattern if the countries are at peace. A random pattern if the countries are angry with each other or new gung-ho officer in charge of the patrol.  Standard customs check points if the merchants are trying anything funny. But at some locations no patrols, no markers, and no way of knowing which side of the border you are on. 

I get your baseline example, but we still have to answer a few more questions.  What kind of technology level are we talking about here?  Railroads?  Until international travel throughout Europe became an actual possibility for significant numbers of people, there was no overwhelming need for border crossing protocols.  If you were on official business, you might carry a letter from the emperor telling the local governors not to interfere with you.  If there were a checkpoint at the road, it'd just be to extract a "tax."  A lot of old countries would have designated free ports where foreigners were just allowed to be, without paperwork, because it was such a hassle checking every sailor.

I mean, the answer really depends on how hard you want to make it.  How paranoid or xenophobic is the king?  If a plague were running rampant through Country X and everyone knew it was just going to have to run its course because the people of Country X refused to engage in any prophylactic behavior, Country Y might very well increase security along their border. Also, if they're not at war, whatever rules they have will be porous enough to allow criminals to smuggle things and people across if necessary.  A ranger could quite easily get everyone over a border by just finding an old goat trail.

Three Four short ideas:

A Great Wall is not without precedent and can be much more epically ramped up, so to speak in D&D Fantasy.

If not a physical barrier, a system of sentries and signal towers (either capable of lighting a signal fire, sending a messenger bird, dispatch riders or magical communication) keeps watch on the borderlands.  If it's a handful of interlopers spotted, skirmishers assigned to the border can investigate, if it's a large army, the dispatch goes somewhere that can muster a counter force.

Diplomacy/trade/espionage.  The country projects assets into the "enemy" country.  These could be ambassadors, merchants, or "spies" embedded into the country, or cultivated by a "spymaster" or handler among natives to the country (what usually happens IRL).  In all three capacities, the agents largely go about their regular business of representing their country to the host government, engaged in trade, or going about their regular job.  If something of note politically (secret meetings are happening, inflammatory speeches are being made denouncing the country that sent the agent etc) or economically (the country used to be a big steel exporter and is now hoarding and military recruiting and ship construction is on the uptick) the agents get word to the nation for whom they work and followup missions may be tasked.   

Proxy war.  Away from the country's back yard, the country covertly supports hostilities against the enemy or an ally of the enemy to keep the enemy's attention or capacities directed away from the country.  

Quote from Kaboom979 >>

This question is posed with two assumptions:

a) the boarder is on a (mostly) open piece of land (no natural or man-made walls cutting off travel)

b) each country has a reason to protect the boarder and stop people from passing freely. Not that the countries are necessarily hostile, but at least cautious. 

When your party cross a boarder between countries, what sort of resistance do they meet? I know alot of the finer details come down to the specific relationship the two countries hold as well as the internal politics of those countries, but I am looking for just some general guidelines. 

I mostly don't use your constraint (a), because ancient and medieval borders tended to be the aftermath of warfare, and thus determined by lines that were practical to defend, and actually determining what side of an arbitrary border you're even on requires fairly advanced navigation.

In any case, this depends a bit on the nature of the PCs

• If you cross at an official crossing point, such as a road, the border guard will be interested to know your business and will likely expect you to pay a tax. Bringing weapons and armor across a border will likely require both money and a quite substantial amount of diplomacy, but should give you some sort of pass.
• If you cross between official points, you probably make it across undetected -- patrolling borders is expensive. However, if you do get spotted, armed and armored PCs will be assumed to be up to no good. Also, trying to enter a town without an appropriate pass will likely get you in trouble.

Few countries in history have constructed walls along their entire border. Most of the time, they'll rely on natural barriers, possibly with a wall along a more passable section. The wall will probably not be manned along its entire length.

If relations are friendly, then there will probably only be checkpoints along roads.

Even when relations are strained, passage of civilians for trade is usually allowed. Walls are mainly to keep out entire armies.

There are several ways to do borders between countries/nations/principalities. This partly depends on how the different areas are structured.

For example ... customs might be much more likely at towns than at the border of a country. Towns are a much smaller area and more likely to be walled. There are a very limited number of ways in or out. If you want to tax merchants and trade it would be better at a town entrance than at a border since the nation likely doesn't want to collect taxes at an outpost that is far from military support.

1) Borders are undefended. Armies and troops are stationed at nearby forts and towns. This prevents military incursions but doesn't interfere with trade across the border. The military is also in strong holds and defensive positions and if an enemy army tries to bypass them they can attack the army from behind and disrupt its supply lines.

2) (1) above + Patrols are run along border regions from forts/towns near the border. The scouting allows the nation to keep track of larger groups crossing the border and possibly deter smaller groups.

3) (2) + more frequent patrols + border check points on roads/rivers/other transport routes.

4) (3) + fortified check points + possibly even more frequent and larger patrols

5) (4) + border fortification - this could consist of watch towers with warning fires in sight of each other and might extend to a complete wall (Hadrian's wall in England and the Great Wall of China, the Berlin Wall, or the ice wall in Game of Thrones would be examples of border walls). However, border walls are very expensive and time consuming so unless there was an ongoing and perpetual threat they would not be a common solution. 

How much border security there would be depends on what the purpose is.

- tax/regulate trade

- prevent individuals from crossing the border

- prevent large groups

- prevent armies

... and on how much the country wants to spend in order to maintain that security. So I'd probably go with a security level of 2->3 most of the time with exceptions depending on circumstances.

Rivers also make good borders especially if you want to be able to better regulate flow of traffic in and out of a country, especially if you want to raise taxes.

In the current world I am building there is only one "contested boarder", between the Orcs and the human kingdom/empire.  Along this boarder the humans have positioned rangers in the large forest between them.  If the Orcs start to venture in, they disappear.

However, the boarder between the High Elves and the humans is not an open boarder.  Both civilizations have positioned a town near the boarder the handle trade.  When a diplomat or government official needs to travel into Elven lands, they travel to the boarder town where they meet an Elven diplomat.  After speaking with the Elven diplomat, the person may receive a "Writ of Passage" that describes where the (N)PC is allowed to travel and how long they expect to be there.  They are then free to travel into the Elven lands within the limits described in their Writ.  They must keep the Writ with them and present it upon request.  There is a well groomed "evergreen orchard" that spans from one side of the peninsula to the other that serves as a clear boundary between the High Elves and the Humans.

Within the human kingdom, folks are challenged when they wish to enter a town or city.  To become a town, the settlement has to build a wall around the town.  Whatever is inside the wall is "the town" and outside the wall is not.  Towns are permitted to expand their walls, so an older town will have sections that have been walled off.  When entering a town or city, adventurers must present their Guild License as members of the adventuring guild.  Possession of a guild license allows them to "carry certain items" in the kingdom.  But that doesn't mean they may carry those items in the town or city.  That is up to the local officials to determine what individuals may carry, and the decision is made up based on the officials view of the threat level, but they may arbitrarily decide "you can't carry more than ..." on a whim, so don't upset the local officials.  Generally, the population may carry a "Bollock Knife" (a small fixed blade - dagger), a club or a quarter staff.  Hunters may receive a license to carry a bow.  "Gentlemen" could carry a rapier and a dagger (sometimes a dagger for the sort that wasn't looking for a fight, a main gauche for a real fighter or a stiletto for the expert dualist).  Members of the town guard could carry their typical arms, and members of the officer class in the army could carry their usual bladed weapon, typically a longsword, but if an officer wished to carry his two-hander strapped to his back, well whatever floats your boat.  Members of the clergy could carry non-edged weapons, so they generally carried a quarter staff or a mace if they needed to carry, but they would also possess a bollock knife and wear it almost all the time.

Adventurers may carry whatever they wished within reason as long as the local officials believed they could be trusted.  Adventurers are expected to use this privilege to assist the local officials if called upon for a fair reward.

Caravans also require permission to operate in my setting, because their cargo could be disruptive, or that they need their own guards, or the officials just find them suspicious.  A lot of bribes are paid by caravans for one reason or another.  The Caravaneers from Colorova sell the best tobacco that can be found anywhere.  They "tribute" a certain amount of tobacco to the local officials in order to maintain their license.  And they carry a substantial supply of tobacco to pay bribes along the way.  But their trade in tobacco more than makes up for the small losses in tribute and bribes.

Diplomats are free to travel with the arms they feel necessary, they only need to declare this on their "Writ."  They are even allowed a small bodyguard.

So the human kingdom has a lot of rules governing the limits allowed for carrying weapons.

The Dwarves abut the human kingdom in the north.  There is no formal boarder between the two.  Of course everything inside the portal to the Dwarven town in the mountain is Dwarven, but they also have a small farming community immediately outside the portal.  No discussions have been initiated between them to formalize this boarder.  The Dwarves figure if they can see it from the portal then they can keep it.  The humans figure they got plenty of land without encroaching on the dwarves.

The Wood Elves also have a Realm that abuts the humans.  They both generally figure the Elves Realm begins where the trees become a forest.

If I needed a boarder guard I would have an outpost less than one day's ride from the boarder on the main road.  This would serve as a supply base for the boarder guards.  The boarder guards would occupy camps that included a wooden watchtower.  The camps would be no further apart than the watchmen in the towers could see.  They would be spread out for a distance along the boarder, but not enough to cover the whole boarder.  The camp at the boarder would have two buildings; a guard "shack" and a small administrative building.  In the building they would sort out whether the group would be allowed in or if they would be told to turn around and head back the way they came.  If they were allowed in, a pair of riders would escort the group in as far as the supply base, where they would be processed again.  At this point they should receive "travel documents" that allowed them to get where they were going.

Regular travelers would have the documents they need already when they arrived at the border.

Diplomatic Missions would be placed in the towns beyond the boarder that would allow parties to get travel papers before arriving at the boarder.  Embassies would be established in the capitols.

Most of this infrastructure would be under the "command" of the Diplomatic Corps, a branch of the government (think of the State Department) mostly handled by Bards, Paladins and Rangers.  They answer to the Monarch/Emperor.  Other major divisions of the government include the Military; an Army and a Navy, and an office of Domestic Affairs and a Group of Religious Advisors.