Although post-ban large-cap mags in MA are illegal for most people, one can't assume that the person who is being questioned about the legality of his mags isn't in LE and entitled to them. Thus I feel that it's rude and out of place for some random person to start asking questions of everyone he sees with large-cap mags on a range. MYOB works best there.
Gotcha and yes you're right, asking anyone is sheer stupidity.