Where to start?
The ring finger is always on the left hand, right next to your little finger.
The average women's ring size is L½
Guessing your partner's ring size correctly can be achieved with a little common sense.
If your partner is short and petite, perhaps around 5'2 (157cm) it's probably safe to assume her fingers are smaller and more slender, so start around a J (US size 5¼).
If she is closer to 5'6" (167.5cm), chances are she has the average finger size of L½ (US size 6).
If she's taller than average or a slightly heavier build, her weight will be naturally distributed across the whole body, so you should be looking at around a ring size N (US size 7).
As long as you can get within two to three sizes of her actual ring size, you can always get it resized, although, a more drastic, like a P to a K, will have necessary costs involved.