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.