I am not an expert, but I'll take a shot.
Gen I = early model glocks have no stipling on the grip and both barrel and slide are stamped w/ little Phoneix type bird stamping. Another quirk of these early models is that the slide release spring is weak, so upon rapid insertion of second magazine, most of these models will release the slide and chamber a round WITHOUT touching the slide release, (the slaming of a mag insertion is enough force to cause the slide release to activate on its' own). They are also known to release prior to secondary mag insertion or in some cases not lock back to begin with.
Gen II = these glock have little nubbings on front and rear strap of grip.
Gen III - these glocks not only have the nubbings on the grip (front and rear) but also have the accessories rail for installation of lights and other devices below slide on front dust cover.
Mike