You know that you're addicted to the Barrow-Downs when you're sitting in study hall, and you're supposed to get your maths done (even though instead of doing that, you're reading a book on Britain) and the computer stares at you in the face. You decide that now would be a good time to check to see if Paper Telephone has started yet. And you are still sitting at the computer...
Also, when you start using BD slang in your diary, and everyday life.