i can very seldom imagine myself topping--and never in a relationship where i'm the subbie. That's why I could never 'work well' if you will, with switches. When my mind gets in its submissive mode there's very little that can shake me out of it heh.
but I dunno, Oz is right, you just know--like you know whether you're gay or bi or straight. It may take some experimenting, but in the end you know--whether you act on it or not.