Do FAs actually know when someone is squatting in first class?

I’ve always wondered this too. You see those posts where someone just slides into an empty first class seat and plays dumb when the real passenger shows up.

Like if someone boards late, sees a couple open F or C+ seats, and just sits there… does the FA already know nobody is supposed to be in that seat? Or do they assume everyone boarded correctly unless there’s an issue?

Edit cracked me up though. Manifest makes total sense. Guess there’s no sneaking around after all.