Imagine that the above person is actually... An imposter! What you must do is back up that claim, through reasons serious... And not so serious. Example:
1: You're an imposter because you're wearing the wrong underwear.
2: You're an imposte because the real (name) doesn't eat soup... With a fork.
3: You're an imposter because the real (person) is actually in my secret dungeon.
Are we clear? Then we may begin.
Oh, and you can only call someone a spy so many times before it loses effect.