Guessing a ring size can be tricky because it is difficult to do without giving the game away. First try asking friends and family if they know or you could get a ring measured if the recipient already wears one on the specific finger. If you can't take her ring away, place it on one of your fingers, draw a line around your finger where the ring falls. As a last resort, you could simply buy a size L, which is the average finger size for the average-size woman. The only way to definitely know is be sized by a jeweller.
You could always take a leap of faith and guess and then have the ring re-sized after the event.