How do I find ring size without the person knowing?

if they wear a ring, admire it and ask if you can try it on and make note of how far down it goes and how loose/tight it is on your finger. You could also ask someone close to them who might know their sizes. Of course, a ring that is gold or silver can always be sized if you bought the wrong size. You could also take a good close up photo of their hand under some excuse or "by accident" as a good jeweler can usually look at a hand and guess pretty accurately at their size. You might also be able to sneak into their jewelry and try on a ring there and remember how it fits one of your fingers and buy one that fits you like that. If they remove a ring, trace around it onto paper when they're not looking.