Borrow a ring they wear on their ring finger and have it measured by a jeweler, or trace the inside of it on paper. You can also ask a close friend or family member who might already know.
Before looking at diamonds, you need to know the wearer's aesthetic. how do i buy an engagement ring
Look at their current jewelry collection. Do they wear mostly warm yellow gold, or do they prefer cool silver-toned metals like platinum or white gold? Borrow a ring they wear on their ring