How do I get her ring finger size?

OK, so I wanna ask her the big question, but I don't want to get hinted at what is going on. Asking her family isn't an option because she's Japanese, her family is in Japan, and they don't speak English. Her friends here are all Japanese so even if they said they could keep a secret they could turn around and tell her what's going on in Japanese and I wouldn't have a clue.

The best idea I've heard is to get a ring she has and take it to a jeweler to approximate her ring finger size. The problem is I've never seen her wear a ring. I haven't gone snooping through her stuff though so maybe she has some.

Any other ideas?

Updates:
+1 y
Thanks for the answers. I actually got passed down a family ring, and as luck would have it, it was the perfect size. I guess keep the answers coming for people who have similar question.
How do I get her ring finger size?
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