It will say Gen4 on the left side of the slide. Gen3 has the mounting rail under the front of the frame
Bumping an old thread here. How do you determine the generation of ANY Glock? I know I don't have a Gen 4, but how do I know if it's 1, 2 or 3?
There's also a Gen3 30 with finger grooves and no rails lol. If worse comes to worse, call Glock up. They're great at answering questions over the phone.