How do you get a ring for your girlfriend without knowing the size?

1)You can just buy it and have it sized afterwards

2) Most rings come new at size 6 because about 1/2 of women are that size on at least 1 finger. Thus you can get lucky

3) You can borrow a ring and find out what size it is already. Then get the ring sized to that size

4) Ask a friend or relative of hers