In addition to what Bill said, I always point the nose of the plane in the air for 5-10 seconds. This tests both the mixture, to make sure it's not too lean, and that the engine is able to properly draw fuel at extreme climb levels. If the speed increases when you raise it, you know it's not too lean, but still could be too rich. If the speed decreases, either when you first point it up, or after a few seconds, you know the engine is too lean, or is having fuel draw problems. If this happens, immediately put the plane down, on the ground, and richen it a bit. Note, I also consider no speed change in the nose up test to be a "too lean" indication. I want my engine a bit rich at takeoff to eliminate the possibility of it going lean later in the flight.