During the preflight inspection, the pilot looked at the fuel level sight gauges and did not see a meniscus point in the clear tubes. He therefore assumed that this meant the tanks were completely full and that the entire sight gauge was filled with fuel. About five minutes after departure the aircraft ran out of fuel, and during the ensuing forced landing the aircraft nosed over on the soft, brushy terrain as the pilot applied "excessive braking" in order to stop before reaching a line of trees. After the accident, the pilot checked his fueling log, and then realized that the fuel tanks had been almost completely empty prior to takeoff, and that the reason he saw no meniscus point in the fuel quantity sight gauges was because the lines were completely full of air. The pilot had not visually checked the fuel tanks during his preflight.