The PHB is pretty vague on how to handle knowledge checks. There's some examples of what some of the INT spells might cover, and I've got a pretty good idea of what skills I'd tie to what type of monster. Where I really feel the rules are lacking is how characters get information about their enemies during combat. I've been running knowledge checks as a free action in my game, but I'm wondering if maybe that's a bit too generous. In the game I play in, the DM has ruled that studying a monster is a full Action, which feels very expensive - especially when I just need to know something basic like monster type to see if a spell applies.

What I've been considering is giving some basic info for free to those with the right proficiency - monster origin/type, some descriptive hints at a poison attack or unusually tough hide - and then more in-depth information available through a bonus action check. My monsters generally have a key vulnerability or other weakness that a resourceful party can exploit, so a good check can be very useful.

How do you guys handle this? Do you just find out what you know at the start of combat? Do you spend actions to study your foes? Do you scale the information given with certain DCs?

I tell my players what the thing looks like. They in turn decide to kill it or not. They rarely ever ask to make a roll to decide what a creature is as it's an action in combat. 

Getting too much info on a creature is a benefit to the players as it can give away some weaknesses or resistances and other stuff. So that's worth a full action in my book. 

In the campaign I run, if a character wants to gather info on a monster they can make a corresponding INT check, and I'll tailer what kind of INT skill based on the type of creature:

Arcana for Dragons and Elementals.

History for general humanoid creatures like orcs, goblins and the like. 

Nature for Monstrosities. 

Religion for Undead and Fiends.

I only let those with proficiency in those skills roll, buy I am always willing to listen to them explain to me how their specific skill can relate to a creature that doesn't correspond to the list above. I don't give out resistances or immunities, unless the group has faced a similar creature before. I do it this way because I have a player that is a Monster Slayer Ranger in my game. To give him more of an important role in the party than just being a Sharpshooter arrow boy, I always let him have more information, including resistances and immunities on his favored enemies. Occasionally I've let him find a monster manual/diary from other hunters to add to his list of favored enemies. So now the group looks to him to lead the party when they're out in the wild, and they look to the paladin to lead when they're in a city or social setting, the bard finds the downtime fun, and the druid is the pet tamer.

I just try to give my players a sense of importance beyond healing, casting spells, or dealing damage. 

Quote from Avohei >>

They rarely ever ask to make a roll to decide what a creature is as it's an action in combat. 

Getting too much info on a creature is a benefit to the players as it can give away some weaknesses or resistances and other stuff. So that's worth a full action in my book. 

 It's worth a full action to you but you say your party rarely ever does it, which implies that it's not worth an action to them. This is the issue I'm trying to fix. If you make someone essentially take their whole turn to identify a creature, they're just not going to do it because that's a boring turn - not to mention if they roll poorly and gain literally nothing for their action. This is how things have shaken out in my game where the DM decided it takes an action - as far as the players are concerned, it has just been dropped as a viable action.

But then you have classic cases where the players know that they need fire to kill a troll, so they metagame that knowledge in there and just happen to finish the guy off with a Fire Bolt. I'm trying to avoid this with a middle ground where there is still a cost but it's low enough to be worth doing.

My other issue with a full action identify is that it just doesn't make a lot of sense in most cases. A wizard with 18 INT who specializes in Arcana would know some things about fire elementals, and that recall would be immediate - he shouldn't need to sit and stroke his beard for 6 seconds before his brain kicks in. Same goes for a ranger identifying a beast or a rogue observing a bandit from a rival criminal group. This is their area of specialty.

Knowledge is a passive thing - you know it or you don't - and it should be instant. At the same time, some information can be situational or simply too useful to be free. This is why I'm leaning towards a two-tiered kind of thing with a free base level of info based on proficiency and more in-depth info at the cost of a bonus action. Cause I know if I make it a full action, my players just won't do it.

Honestly... I allow a check depending on the type of creature (whether arcana, history, nature, etc. would make the most sense) as a free action during combat if a player requests it. I don't think this is ideal... but I do this because I have a Wizard player in my current campaign who panics a bit and second guesses herself a lot. So usually if she's asking for a check on that it's because she wants to make sure she's not going to waste a spell slot casting something the creature is resistant or immune to. 

Outside of combat if someone makes a solid check I'll essentially read them the creature's description. If it happens in combat I basically ask her what she wants to know... like the creature's weaknesses or resistances and I'll reveal that much from a free check.

It's worth a full action to you but you say your party rarely ever does it, which implies that it's not worth an action to them. This is the issue I'm trying to fix. If you make someone essentially take their whole turn to identify a creature, they're just not going to do it because that's a boring turn - not to mention if they roll poorly and gain literally nothing for their action. This is how things have shaken out in my game where the DM decided it takes an action - as far as the players are concerned, it has just been dropped as a viable action.

Good point! They are long time players and meta game the monsters a lot. I don't mind it. 

Maybe I'll change it to a bonus action. Would be more fair.