How do you tell if a guy likes you even though you don't see him that much but you know him well?
Tell him that someone has asked you to marry them and you wanted their opinion on the matter. Look closely at his reaction and response. If they oppose the union then they might like you. If they think that you should just say no then ask why. If they think it is great idea then they probably aren't very interested in you. If they start to plan your wedding then they are probably gay.
On the other hand, you could simply be honest with the person and ask them if they would be interested in getting to know you better. The worst case scenario they look at you as if you're crazy and you can just laugh it off.