See this picture:
http://i106.photobucket.com/albums/m254/duggan12/563decfb.jpg
Notice how the first three magazines, the "9mm" marking is halfway between the top of the witness holes and the metal liner above? The last magazine, the "9mm" marking is much closer to the metal liner above? That's the one true reliable way to know.
If the caliber marking is halfway between, it's pre-ban. If the caliber marking is higher up, it's post-ban.
The earliest 9mm magazines didn't have a caliber marking because Glock only made 9mm handguns and there was no need to distinguish between calibers. If you have a magazine that's not fully metal lined, has no caliber marking, but DOES have the Glock logo, it's a pre-ban.
Also, some of the later production pre-bans actually had the higher caliber marking because Glock was anticipating the Assault Weapons Ban. But there's no way to distinguish between such a magazine and a post-ban, so if the caliber marking is higher that magazine should remain in a state with no capacity limit.
