How to tell if a ring is stainless steel and doesn’t contain copper?

I’m broke, 24 and am about to do the least-traditional way of a proposal to my partner (as were not legally allowed to be married) of 10 years and want to get a ring that doesn’t turn his finger green- he's sensitive to what im assuming is copper? In my understanding, it’s the copper present that turns someone’s skin green with rings/jewlery?

I’m wondering if there is a way to tell if something is stainless steel- is it just the magnet method? Can one notice visually if there is copper within a ring band?