Photo guides like the one Badger linked above are your best initial source of telling good gear from fakes. The reproductions as made won't fool serious collectors, but the fakers have become quite adept at enhancing things. So you have to lok for clues in the shapes, machine marks, markings, etc.
I almost bought a "repop" scope just for comparison, but I'm not working a blue million hours of OT this year, so...