The perfect step that you can take is this one. You can simply ask your girlfriend about her ring size. This will also determine two things. First, you will know whether she wants to marry you or not. It will be an indirect way of approaching her, and she will also give you a signal whether she wants to marry you or not. Just approach her and ask her about her ring size. If she tells it to you willingly, then I think you will not have a problem.
If you don’t want her to know about the fact that you are trying to find a ring for her, then I think stealing is the second option you have. Try to steal one of her old rings. She will barely look for an old ring, and you will be able to find the correct size. However, make sure that the old ring still fits into her finger. It will really help you get to know about her ring size. It is a difficult technique which will require you to know a lot about her old jewellery, but it will definitely help.
Friends and Family
You can simply ask her friends or family. If you are about to propose to her, it is obvious that you have already spoken to her family about it. Now is your time to shine, and time to ask her siblings, or parents, for her ring size. For sure they will also try to make it easy for you and will easily tell you what her ring size is. Moreover, best-friends are of great use on such occasions. Ask her best-friend and see whether he/she is willing to help you. If they are, that means they also know that your girlfriend is ready to accept you in the future.