If you’re planning to surprise her with a ring, asking her “Um, what size do you wear?” is one of the more efficient ways to ruin the surprise. It’s possible that she may have acted proactively, so to speak, but you can’t count on that sort of thing. If you happen to be close enough to Germany, this seems like a nifty way to get the information you need:
It’s generally hard to make an impression on a piece of cardstock that’s 3.5 by 2 inches, but German agency Jung von Matt definitely found a winner with its incredible business card for jewelry company Marrying which, as the name suggests, specializes in engagement rings and wedding bands.
The card rolls up, becoming a handy tool to measure one’s ring size. The idea is that men who are shopping for a ring can use the card at home to subtly check the size of a woman’s current rings.
Assuming this can be done subtly. I’d probably run the risk of “What the heck are you doing in there?”
(Via this tweet by former OKC jeweler Dan Gordon. He’s still a jeweler, but he’s relocated to another state starting with O.)