"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)
HER: "Nooo???"
"Are you sure?"
HER: "Yeahhh"
"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.