I'm afraid I wouldn't take the ring that was proffered to me if the customer just licked it. I'd just keep my hands to myself and look at them. Perhaps if you did that a few times, people would think first? Nah, they wouldn't, but it might make you feel better.

I would keep several bottles of germ x prominently displayed on the counters where the rings are kept, and if a customer licked a ring and then offered it to me, I'd germ x first.

Gale