Easy to tell the repro when you have a real one next to it for comparison, but you don't always have that luxury.

That is why I prefer to look at the backside. As Huskerscott alluded to the real ones should have some texture to them. Repros are usually smooth as a baby's butt.

Second test is to try to chip off a little bit of the paint with a key or something. An original fired lens it will be very difficult to chip the paint.

-Vern