usually when this happens to me it is someone I have known for awhile. So, my response would ve.. " you have known me for how lond and your just figuring this out now?? A little on the slow side, aren't we?" or just a resounding chorus of "DUH" for the minons works well also..
if it is someone I don't know.. Well, they usually don't say anything, which is met with a "thought so" sort of smirk with a side of stink eye. -shrugs-