How do you prove someone stole something from your house? I took off my ring and earrings (earrings not real) ring real beautiful blue topaz and put the all in the same spot. The next day they were gone!!!
This is a long shot, but maybe that "...Always goes back to the scene of the crime" quote will help you out here, like if one of those girls looks or gravitates near that spot. A sudden nervosity around you would be a good indication as well. Failing that, I'd wait till error on that one's part occurs as most amateur criminals often do slip up.
Disclaimer: I'm not a criminal expert, simply a neurotic.
I'd get a camera and set them up ..lol you could use another ring or something .
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