The version I've done before involves having her stick the card into her bra on a minor trick with just her, or her and a couple ppl.
Then you move onto another trick, get more ppl, gather a crowd. Most of the time they just leave it, forget about it, whatever.
If she still has it in there once you have the larger crowd, you score gold. If not, then you never mention it. The good reaction comes from her being embarrassed that you're singling her out in the crowd.
INtresting. but still. He had her pick out a card remember? She would have been. oh hey thats the card i picked last time! or w/e i think thats a good way. but how are u gonna get her to forget? lol