"Does this belong to you...?"
(Setup: Before you try this you need to preplan and have a piece of jewellery or something with you. IE... bracelet, necklace, etc even if it's plastic junk)
"Excuse me does this belong to you?"
(hold out jewellery)
"Are you sure?"
"Oh, well I found it at the other end of the bar and the guy there said it belonged to the prettiest girl in the bar, so I figured it was yours"
works 100% of the time.