How do I learn my girlfriend's ring size?

Since my girlfriend's birthday and Christmas are so close I usually buy her gifts for each at the same time. Now for my mom and sister, I usually buy earrings because they're easy and it's all good. My girlfriend doesn't have pierced ears which means I am stuck with necklaces, bracelets, and rings as far as jewelry goes. Unfortunately nobody (me, her, nor her mom) knows her ring size (s). How could I discover this for ring gifts as well as an engagement ring (eventually)? I do and can buy sexual gifts, but those can't really be opened in front of our families. So until she's not showing her family the gifts I got her (unless they're like panties and bras, I don't care if her family sees those that I might buy for her) I have to buy appropriate things, and rings work (when they fit).
How do I learn my girlfriend's ring size?
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