Borrow One of Their Rings: If the person already wears a ring on the same finger you intend to purchase the ring for, borrow it temporarily and take it to a jeweler for sizing. Ensure that the ring you borrow is one they currently wear on the intended finger.
Ask Their Friends or Family: Enlist the help of close friends or family members who might have this information. They may know the person's ring size or be able to discreetly find out without arousing suspicion.
Trace Their Finger: If you have access to their existing rings and they won't miss them temporarily, trace the inside of a ring onto a piece of paper or a soft material like soap. Take this tracing to a jeweler for sizing. Be sure to use a ring from the same finger you intend to buy for, as ring sizes can vary between fingers.
Remember, it's important to be as discreet as possible when attempting to determine someone's ring size secretly to maintain the surprise element of your gift.