What's that old saying?

Driving through our neighborhood on the way home, Spencer announced quite out of the blue, "I don't judge people by their houses, I judge them by the cars they drive."     I reminded him of the old adage that you cannot judge a book by the car it drives (or something like that). When questioned, he assured me that he did not mean it at all and that he does not judge people either by their cars or by their houses.