Changelings

[Laserlight]

Alice and Bob are both changelings, and both are in human (or "non changeling") form when they meet. Can they each tell that the other is a changeling?

[Falcon X]

I just skimmed the 3.5 edition book "Races of Eberron" which is likely the first major place they were detailed at length. It says nothing on the subject, so I assume they perceive each other as any other race would.
They are however, gifted at all things social, including Insight, so they would likely be better than the average person at reading the necessary body language and other things to figure out impostors.

One of their racial traits explains:
They get bonuses to Bluff, intimidate, and sense motive because they are inherently gifted at deception and intimidation, though they cannot actually detect thoughts like dopplegangers can. However, they can intuitively read body language and attitude with surprising accuracy.

[Daithi]

I haven't seen anything that indicates a Changeling can recognize another Changeling when in changed form, so I'd say no. They can't recognize one another. But, I wouldn't argue if a DM played it just the opposite.

[Lunali]

At most I would make changelings more likely to suspect something is amiss. Though they'd still have to contend with each other's deception checks, made at advantage.

[Finback]

At most I would make changelings more likely to suspect something is amiss. Though they'd still have to contend with each other's deception checks, made at advantage.

This, exactly. My thought was "do they WANT the other character to know?" I'd have either or both make a Deception check. Likewise if they're seeking to determine.

eg Alice is trying to be covert, so she gets advantage to deception IF Bob isn't actively trying to discern.
or Alice isn't trying to be covert, and Bob IS trying to discern if she is, so he gets advantage on his Insight.
If neither is trying at all - like two random people meeting at a party, I wouldn't bother making them check. A check should only be when it's relevant to the story.

[Markoff Chainey]

We use a rule for changelings (and for similiar situations as created by a disguise kit or illusion spells, ...) - that whenever a person is in contact with an impostor and that person has a deep knowledge of the original (family member, close friend, etc) or has reason to doubt, the person will make a check whenever a doubtful situation arises or at least once per day.

We handle it like a "death savings throw" - after either 3 successes or 3 failures, whatever comes first, the person is either totally convinced that the imposter is the original or is convinced that there is something amiss and that person is not the original person anymore (not necessarily that it is a changeling, doppelganger, though).

The check is made by the NPC against DC calculated from 8 + CHA (from the imposter) + or - situational bonus; advanatage for very close persons, disadvantage if the person didn't really know the other, disadvantage if the imposter has indepth knowledge about the relation of the two, ..

This has proven to create a more consistent experience for these cases.

[MightyDuck]

As a long term changeling player I can say definitively that the answer is no, at least not in the circumstance you described. The changeling's disguise feature isn't an illusion and thus cannot be seen through in the same way.

In a vaccum it is impossible to tell if someone is a changeling, they aren't impersonating other people and they aren't being deceitful, changelings express themselves through their many faces, their different forms aren't disguises but identities that the changeling creates.

The only way a changeling would be revealed would be if; an external element who is somehow aware that there is a changeling present, the changeling drops unconscious, chooses to use its unsettling visage feature or if the changeling chose to reveal itself. Changeling don't have a special sense or feature that allows them to detect other disguised changelings and could only determine if someone was if their behavior prompted an insight check specifically to determine if they are a changeling.