Some signs, although this is NOT AN INCLUSIVE LIST - eg, there may be LOTS more prebans that don't meet these criteria:
-Mags have zig zag or zipper pattern in the back.
-Mags are phosphated (dull grey or black coat)
-Mags have witness holes in the back and not the sides
-Mags DO NOT have any LE/Mil Marking on the front strap of the mag (this is pretty much the main issue, anything with this marking is illegal for you to own, for certain)
Also be aware that there are vendors like Eagle, Ramline, MecGar, etc, that made prebans for these guns, and IDing the difference between pre and post on these vendors may be pretty much impossible.
-Mike