I want to marry my boyfriend

I know this isn't common but I want to ask my boyfriend to marry me. I just don't know what size ring he wears since he doesn't usually wear rings. So how do I find out what size he wears without actually asking him.? We've had discussions about marrying each other we've also discussed if I would ever propose to a guy before we got together and if he remembers then he's going to get suspicious if I ask him what size ring he is since I said that I would probably be the one to propose anyhow. He and I have seen no problem with a woman asking to man to marry them so I know it wouldn't hurt his ego or masculinity. Anyways, any ideas on how to get his ring size? Cause I do want to actually get him a ring.

No judgement please and thank you