Hi

A friend of mine is thinking about playing a changeling

I'm questioning myself how i would handle some situations...

How is the community detecting that a changeling is present?

Are they accepted in most community?  Orcs aren't except the bigger and more mixed cities, how about Changeling?

how much of a wild card is this race.... can i steal, stab, kill, infiltrate and as soon as i can just jump a roof down in the street, i studently change to whatever and i'm free to go?

It would be up to the DM how they're viewed since Changeling do not originate from FR but from the EBERRON setting.

Personally, i'd rule that in FR Changeling would be quite rare and generally confused as dopplegangers and accepted in similar ways, meaning they'd be viewed as shapeshifter frequently used as spy. I tend to not include specific setting's races as known races in other ones but more like foreign outsiders from different world that are not commonly known.

Same here

MMM presents Changelings as from the Feywild now and not bound/based to Eberron.,..

Anyway, had Changeling in one of my games for a couple of levels before they had to shuffle into a different group.  Game was loosely set FR, actually "post FR" (multiversal cataclysmic event has shaken things up, and the party actually met them, the changeling, in an ale hall in the Avernus demilitarized zone. Changeling's back story was literally amnesiac.  I ditched the Feywild mention and had, and still plan to make further use of the Changeling as the template/basis for an adversary culture I pulled from Tales of the Infinite Staircase (2e Planescape adventure) that basically developed elaborate magics relating to mirrors which it had largely retreated into.

Anyway that lore aside, the character was pretty fun.  Generally worked as "the face" or maybe 'the faceless' for the party.  The basic M.O. would be the Changeling would develop a mask for a particular infiltration, reconnoiter as much as they could, then report back to the party.  Had Charlatan background and mostly levels in Rouge, and pushed their social engineering espionage skills. Player was very good at chewing the scenery (think Donald Sutherland in the Dirty Dozen during the war game, player had never seen the movie but basically pulled off that scene, in Hell).Some reigns I put on the shapeshifting that do really conflict with the RAW, and some actually come right from it:

• Only the body can be altered within the shapeshifting, clothes/gear don't do the She-Hulk trick, no spandex in D&D, so if you change your appearance beyond say skin tone, hair and facial features, they're clothes will look "ill fitting."
• Shapeshifter, not voice shifter.  They could try to disguise their voice, but that's a skill test not something granted by the shapeshifting.
• Shapeshifting could try to "doppelgänger" someone, but there power doesn't innately give the shifter the ability to produce perfect facsimile appearances of a specific individual.  In the game, the character had high deception and proficiency in a disguise kit, but DC checks had to be made at difficulty set by a given NPCs relationship with the imitated.  
• Masks could be made quick, but masks could also be seen through by the perceptive.  More convincing personas had to be practiced till it "sticks" to the PC.  Think of Tim Roth's "commode speech" scene in Reservoir Dogs where the cop "becomes" a drug dealer.  Those were harder to recognize, but again they took time and practice to develop.

Like others, folks like this character were very rare.  The civilization they don't realize they came from is actually incredibly xenophobic and really doesn't venture out of the space its created for itself.  Keep in mind, this was also planar hopping game and a lot of extraplanar types are accustomed to shapeshifting and true sight type capabilities are not uncommon among the planar hoppers.   In FR, I'd encourage the "slippery" mask and more "mastered" persona distinction.  The difference is touched on in the race description but a DM can really lean onto that distinction to prevent some sort of OP polymorphously perverse character with perpetual immunity to discovery in crowds.

I personally would like to try a changing both as a player and as a DM

In modern lore changelings and doppelgangers are typically feared and often portrayed as evil.  Which I think is wrong, but then again I don't think all red dragons should be portrayed as chaotic evil 

I would think the only way to determine if someone is a changeling is if you see them change. A changing, unlike the disguise spell, can with stand close physical examination. But not questioning .

All in all I think it would be a difficult race to play and run. 

Changelings were seen and portrayed as evil because they replaced your children with fakes. That has probably transferred into other contexts as well. It's not the same situation as dictating that a dragon has to be evil.

As for detecting them, there are also behavioural cues that other people can pick up on - cues, or lack thereof that a changling wouldn't replicate. Unfortunately, 5 mechanics isn't very well constructed for such intricacies.

There isn't a system for spotting fakes but I think the system could handle it easily with skill checks at the DM's discression. Deceptions on the part of the changling, insights against them, maybe perception or investigation to notice flaws in their disguise etc. 

The Disguise Kit Tool proficiency discussion in XGTE definitely helps ball parking DCs and the sorts of tasks a Changeling might be faced with when presenting a Mask or Persona with the intent to play a lie or otherwise misdirect.  

But to the OP's more lore oriented point, I believe Changelings would be rare in the FR, with the exception of those locals with strong ties to the Feywild (if you go by MMM retcon).  They'd likely be highly valued mercenaries by various types of powerful people.  The existence of one amongst a community might disturb said community, of course if a Changeling stuck to one persona while home and only shifted to a mask or other persona when away "on work" or what have you, no one would be the wiser.

Also thinking of an individual Changeling playing a "game" with their locale playing two different personas, both older than anyone else in town.  They hate each other, which is why they're never seen in the same place, the hatred stemming from some instant no one else is old enough to remember.  Through this antagonistic dynamic, the Changeling is able to gather a deeper understanding of their community than any single "individual" ever could.  

That is some interesting ideas.  I'll make sure to keep those in minds and check that XGTE Disguise Kit mechanic

Thanks everyone :)