There are a few options for determining this, some sneakier than others. Take a ring she wears on her ring finger to the jewelry store, advises Rose. Once there, the jeweler can use a ring sizer—that's what you see to the left—to find her size. If there’s a chance she’ll notice the missing jewelry, request a ring sizer from a jeweler (many will send it to you for free) and measure one of the ring-finger rings on her dresser when she’s not looking. Or ask a trusted mutual friend to find out for you. Worse comes to worst, take a guess, erring on the bigger side, and get it resized after you pop the question.