It all depends on how good the repair person was at their job, and the quality of the materials they used.
If he didn't do a lazy slipshod job at it, and he used parts from another iPhone, it might pass for genuine and you'll be good. Also, it helps not to act conspicuous about it.
If the parts used were cheap knockoffs, or the repair is shoddy, they'll know right away.
I've been at at the Apple store and seen several times where an Apple Genius has broken the news to someone: "i'm not saying you did it, but, there are non-genuine parts in this phone and we can't service it." Then the owner acts dumb/flabbergasted/angry.