Here is a list of strategies we hope will be useful:
If she has a ring that she wears on the correct finger and leaves it off, even for a moment
1. If you can bring one of her rings to the jeweler without arousing suspicion, this will probably be the safest course of action. Offer to take her jewelry to be cleaned and have the jeweler measure her ring at the same time.
2. Put it on your own finger. Push is down as far as it will go and mark the spot with a pen. Show that mark to a jeweler who will measure it for you.
3. Trace the inside of the ring on a piece of paper which the jeweler can use to determine the size.
4. Make an impression of the ring in a bar of soap, a piece of clay or even a piece of bread.
If she has such a ring but never seems to take it off
1. Buy a box of pizza mix and have a pizza night. Get your hands both in the dough. Now the ring she is wearing must be removed and cleaned. Offer to have it done for her.
If she does not have a ring for the correct finger
1. Tell her you are buying a simple ring for your mother, sister (etc) and ask for her assistance in picking it out. While at the jeweler's find out her size.
2. If you think she will suspect this, one of her friends can be of assistance. It’s not uncommon for one friend to ask another for their ring size as a matter of curiosity. Just be sure you choose a friend who can keep a secret.
3. If you don't trust her friends, employ one of your own who can keep a secret.