I've been lucky to have had an amazing professor in college who has since served as a mentor and played a large part in helping me think through inflection points in my career. At one point, not too long ago when I was consulting him about a career decision and going through my analysis, he said something along the lines of "well, there's always the shower test". "What would that be?", I asked. It's what you think about when in the shower (or when your mind wanders), he explained, and if you're consistently thinking about something else, that is a signal that you should pay attention to. It was a great piece of advice, and it helped me make my decision. I was reminded of this when reading Paul Graham's essay on The Top Idea in Your Mind (hat tip to Pierre), where he refers to what you think about in the shower. It's a worthy exercise – consider what you think about when in the shower.