Well... if we are talking about 4th gen PO, a few things worth taking notes:
Crown is totally different. See picture:
Rep crown
Genuine Crown
Clasp comparison. Seems that the rep clasp on 4th gen does not close properly. Notice the finishing too. Left hand side is genuine.
Second hand tip. Look at crick's picture. The genuine has it's own glow that the rep will portray a duller colour.
Pearl... This one is the dead giveaway. Mine is fake pearl. So refer to crick's picture again.
Luminosity... Without doubt, genuine wins hands down:
All in all, the finishing on the genuine is better (what do you expect? You're paying 10 times of the rep price over here :lol

.
AR coating...
Taken from the same angle with the genuine, you won't see the same reflection (it'll diminish drastically).
Let me reiterate my point: The rep is well done and worth the money! I can't deny that but still, the genuine is definitely up there... If not, you ask crick, why did he spent 2000USD+ on the genuine. It just has this X factor that you won't get from any rep.
And if you're really anal about the crown, 4th gen has the screw thread outside the crown tube. The genuine has inner screw thread on the tube.