Adiamondisforever.com provides this list of stealth methods, which I am paraphrasing and reviewing for you here ...
- Make an impression: The folks at ADIF suggest picking up one of the rings that she leaves lying around and using a bar of soap, piece of clay, or even a piece of bread to make an impression which can be measured by a jeweler later. My thoughts? I do not actually agree that this would be a reliable method. Clay and bread bend and stretch, and are you really going to take a bar of soap into a jeweler? If you do go this route, at least make sure the ring you're impressing is one she wears on the left ring finger.
- Put it in writing: A step up from the impression in a slice of bread, ADIF suggests that you trace the inside of her ring with a pencil to take in for measurement.
- Try it on for size: Try one of her rings on your own finger and mark on your finger how far down you were able to get the ring so a jeweler can help you measure the size. I don't have a problem with this method, but I'm still not entirely convinced it will be accurate.
- Call on one of her girlfriends: Guys may be surprised to learn that ring sizes do sometimes come up in conversation between women, so if you can get one of her friends to casually ask her, she probably won't suspect a thing. Maybe so, but be sure the friend has an answer ready when she says "Size 6, why do you ask?"
- The real thing: Methods 1-4 are creative, I'll give them that, but far from foolproof. If you want to be sure to get it right on the first try, your best bet is to bring one of her rings to a jeweler for measurement. Again, remember to bring a ring that she wears often (so you know it's one that's comfortable on her) and on her left ring finger -- almost all ten fingers are different sizes, so this is the only way to be sure. One last thing to consider - the thickness of the band will make a difference. Bring in a ring that is about the same width as the one you are proposing with.