Do NOT ask her girlfriends—they will not be able to keep the secret! Lol
Does she wear a right hand ring? You could like take it off when you’re holding her hand or w/e and try it on yourself. (on your pinkie) remember how far it will go onto your finger (mark it later) and then go to a jewler and ask to try on their ring sizer. You should be able to guess pretty accurately.
If you guys are REALLY close, you may be daring enough to check her jewelry box. (do NOT do this if there is any risk of her catching you though! Lol) find a ring of hers (that she wears on her ring finger) and check for the size that’s marked on it.
Take her bone structure into consideration. Is she skinny? Average? Large? Small or large hands? Most women wear between a size 4 and a size 9.
There is also supposed to be some kind of correlation between shoe size and ring size, but this varies quite a lot. I happen to wear a size 7 in both shoes and rings, but this is not true for everyone.
Best of Luck! I hoped this helped. Try number 1 especially!
Do you live with her? If so you could measure her finger while she's sleeping. If not I'd go to the store and ask them if you get the wrong size if they can resize it for you and if they can I'd buy a size 6 because that's a medium size. If they can't then I'd propose to her and tell her you want to go shopping with her for the ring to see what she likes.
when you are at her house sneak a peek in her jewelry box when she's not around and grab one that she might not notice is missing. or if she's got siblings ask one of them to do it and give it to you. otherwise most rings on display at a store are around a size 7 and that fits an average size girl. you can always get it resized.
My roommate told me about a card she just found on Etsy.com. It is a really cute card that lets your girlfriend input all her body measurements (including ring size). Give her the card and ask her to fill it out claiming it would be a really helpful guide when trying to buy her something silky for Christmas... anything to throw her off the scent.
It's called the My Digits card - find it on Etsy in the shop called PulpLove
How I did it. I cut a piece of cardboard into a triangle. I waited for her to take her favorite ring off and slid it onto the triangle and marked where it stopped. Be sure to note if it is the underside or the top side of the ring. I then took it to the jeweler and she measure it. Was pretty simple and the engagement ring came as a big surprise. Good luck!