Originally Posted by -Ralph-

Get out your 1/4 inch drive socket set (the wee one with the sockets about the same thickness as fingers) and try her existing rings on the sockets until you find the diameter socket with the best fit, then take that socket to the jewellers and tell him that's her ring size. He'll laugh his head off, but it works.

Alternatively if you can pinch a ring when she won't notice, the jeweller will measure it on a taper for you and tell you her size.

If there's any doubt get a size too big, easier and cheaper to cut and make a ring smaller, than add more gold to make it bigger.