Mags made during the ban have "LE and Military use only" stamped on them if they are designed to hold more than 10 rounds. Most (if not all) mags made since the ban expired in 2004 have no such markings and for most of us any mag is now legal. Even the LE only marked mags. I have no idea how people in states with a 10 round limit can prove if it was made prior to 1994, or after 2004.
If anyone living there can shed some light on this I'd like to know how it works.