You trip over something in public, look up at your friend and say, "Sorry I just failed a Balance check," and he replies "What DC?" and you answer "25."
When you go out to drink with your friends on a Friday night and you specifically ask not to have anything game related come up, yet they still spend most of the night talking about feats.
People start referring to you as "the Dungeon Master" when describing you to people who don't know you.
