How to secretly get ring size??

For Christmas I decided to get my boyfriend a promise ring, the only problem is I don’t know his ring size!! I have a size 10 ring that he put on before and I think he said it fit but I don’t remember for sure. Is there any way I could secretly ask him his ring size? He already thinks I’m getting him a promise ring but I don’t wanna go and spoil it because I wanted it to be a surprise.