You can't. Unless it has a serial number on the mag to prove the date it was made or is stamped with the restricted to LE only there is no way to tell when a mag was produced. Unless of course it is for a gun that was released for sale after the 1994 ban so then in fact there are no preban mags made for it. It's a joke. Every hi cap I own is a preban.
NRA Benefactor Life Member