Interesting idea, of course it seems very unlikely, but running with it, what could potential tells be? I imagine there might be tells something weird is off if you got really close to them, so they may stay on the periphery of most humans lives and largely stick to their own (definitely at least for mating).

I'd bet you could tell with DNA analysis, or through other medical means such as scans since they wouldn't have had time to adjust for that. If even a single mimic was found the secret would get out, so they'd have to be very good at avoiding the medical world.

I guess you'd probably be looking at either small groups of particularly successful people who have the resources to keep their mimicry hidden and keep to themselves (nobility?), or at some societal outcasts that roam around, never getting close to anyone.

They'd somehow have to avoid their nature being discovered even after death which may be one of the harder things to believe, since presumably they could also die suddenly from accidents etc. which would often prompt careful medical examination which would then reveal the truth. Of course it's possible some individuals did discover the truth, but no one believe them, but it's hard to imagine that at scale. So if these mimics exist, we're probably talking a few hundred or thousand of them at most