If you are a Pizza delivery guy (with all due respect, it's just an example) and you wear a gold patek, people who know you, and know watches, will definitely know that it's a rep, even if you inherited it and it's 100% gen.
If you work for a law firm or have a good job and wear a 10$ beach fake, people will expect it to be real, and won't even question it.
This is if people know your background, so like family, friends or friends of friends.
The biggest tell of a rep is you.
If you are talking about strangers then, unless you are wearing a rep of a 900k watch or something extreme, they won't give a shit, unless they know watches (or reps) a lot, there is NO WAY for them to call you out.
BTW, no one will call you out, people would "suspect" at best, but won't openly say it