Some people claim that this is an exact rep, it's not but it is incredibly well done. For 3% or so of the price of a real one it is incredibly well done.
These are how I tell them apart (I just got a black dial JF for comparison):
The gen's bracelet doesn't have as much play and the rep's bracelet has a lot of sharp edges. You can't tell until it's in hand however. The "Flash" of the JF's bracelet is VERY comparable to gen. Realistically, IMO, this is very fixable with some more finishing of the band IMO.
The dial is a tell but only up close, the squares on the gen dial have, roughly, 10 lines per square and the rep has like 5 lines per square. This is only a tell with a loupe; on the wrist the "flash" of the dials are comparable.
Height of the case but we knew that already.
AP logo on the bracelet is a bit off, also the AP logo square on the clasp sits a bit further above the bracelet itself.
Crown has a sharp edge on it. The AP logo on the crown, when screwed in, is not at a straight up 90 degree angle. Now, whether my gen does this and not all do, and vice versa on the rep I am not sure.
Overall it's a phenomenal watch for, what, $3-350? The cheapest I see any legit 15400s selling for is around $12,000 or so.
Oh yeah, one last thing... In real life blue dial 15400s are very rare beasts, might want to consider that before buying one. I see 15400s in the wild somewhat regularly but have NEVER seen a blue dial on someone's wrist.