Post Ban large capacity mags were marked "Law enforcement only" until the Clinton era ban expired. They are no longer so marked.
Any large capacity mag manufactured during the Clinton ban were marked and are definately post ban. NY now has a problem with their AWB, none of the new mags are marked so they will have a hard time proving they are post ban. (remeber, they have to prove they are post ban, you do not have to prove they are pre-ban)
Not talking NYC now...but, it is my understanding, if the weapon was manufactured before the ban, and the mag does not have "law enforcement only", I don't think they could prove that the mag was manufactured post ban and it would have to be considered to be legal. It's a stick wicket. Doesn't mean they can't/won't charge you..just means I do not see how they could convict you.