I don't see when that would be relevant to a casual conversation.
whenever there's a lull in the conversation, you could mention some guy you heard being a jerk to someone because they were gay (even if you had to make up the story), and the way your friend reacted would let you know how he might feel about gay people. learning a friend of yours is gay could be a bit of a shock, I guess, but at least you'd know that, if you told them, they wouldn't care in the long run
if it went well, you could just go ahead and tell him you were gay then
if it did NOT go well, stop talking to that friend, and find new ones