Question: how do you know what size ring your girlfriend wears?
Answer: “Borrow” her jewelry. If she already wears a ring on her ring finger (even if it's on the right hand), you can use that as a guide. ...  Phone a friend. Enlist her best friend, sister or mom to help you. ...  Hold her hand. Next time you're holding hands, pay close attention to your entwined fingers. ...  Put a string on it. ...  Give a guess.