Many times you will see tour professionals walk up and look into the hole. They are actually looking at the cup liner and soil just above it. If one side has more soil showing above the liner, it means that a putt will break away from that side as it nears the hole.
Also, if the grass on one side of the hole is cut shorter than the other side, it means that the cup is higher on that side and the ball will break in the opposite direction. You can learn a lot from peeking into the hole when you putt.