You can start by just asking her. If she doesn’t know, continue reading and find out how to get it by yourself.
Keeping the ring a secret? Then you’ll have to get crafty. Secretly "borrow" one of her rings (making sure it’s one that fits the intended finger) and compare it to the size chart, or you can have a jeweller size it.
Slip it back before she notices.
If you can’t sneak her ring away, try this: slip it on your own finger, as far down as it will comfortably go, and mark the line with a pen. A jeweller can then use that mark to measure and determine her ring size.
When in doubt, err on the slightly bigger side. It’s easier to have a ring sized down than up.
Caution: it is important to compare rings for same finger and same hands on methods 2 & 3.
Note: For accurate ring-size results, you must make sure that you do not select the option to fit to page or to shrink to page.
Your street jeweler has all you need to know your ring size or the size of the person's finger to whom you want to gift a ring (you need to have "borrowed" a ring from this person before).
A ring sizer is a group of circles of various sizes. It is the most precise tool to find your exact ring size. Try the circles on as if they were rings. When you find the one that fits the best, just read the size on it.
This weirdly named tool helps measuring the size of a ring and not of a finger. Here, you need to have the ring. You will then find the size. Pass the ring on top of the cone and let it slide all the way down the triboulet. When the ring is blocked and cannot go more down, just read the size where the ring is blocked. You will then know the size of this specific ring.