The easiest way to determine your partner's ring size without asking them directly is to take one of their rings they already wear to a jeweler who can measure it. If you don't have access to a ring they already own, you can trace the inside of a ring on a piece of paper and measure the circumference of the circle to estimate the size. If you don't want to take any chances, you can also get a plastic ring sizer from a jewelry store or online to get an accurate measurement.