Frederick Buechner Quote
By and large a good rule for finding out is this: the kind of work God usually calls you to is the kind of work a) that you need most to do and b) the world most needs to have done. If you really get a kick out of your work, you've presumably met requirement a), but if your work is writing TV deodorant commercials, the chances are you've missed requirement b).
Wishful thinking: a theological ABC. (ed. Harper San Francisco, 1973)