some details off the top of my head:
-real shure mics will comes with a warranty sheet and a satin finish sticker as well as a leatherette bag. fake ones will have neither and will have a "pleather" bag instead.
-real shure sm57s have NO bevel on the bottom of its casing where the XLR plug goes , this part will be one solid part. the fake ones have a 2 step bevel on the bottom of the mic.
-the real shure mics are heavy and the casing is colored much darker in comparison to the fake (greyish color)
-the real shures come in a glossy box with a hologram sticker to indicate autheticity. sm57s come in the bag, there is no hard case for the 57 like you see on the SM81 and the other more expensive mics, just a nice bag.
-NO real shure comes in a cheap mass produced box and there are no OEM versions of the mic which can get sold in a cheap box.
if in doubt, borrow someone's real sm57 and bring it along. the differences are very obvious...that is unless the bastards have gotten better at duplicating them.