how do you find out that you are already having an affair?

Affair like cheating or affair like having a romantic relationship? Well if you're feeling something for a friend then i guess you should just be honest about it. Start slow like tell him you like him. I don't think that will create a gap when you take it slow. If you're going to shock him with your feelings, then most probably, there will be some tension. The best way to do it is to act normal and do you thing as close friends.