The 2 most basic things to check: 1. Remove the glow plug and look for a bright glow. Inspect the element for breaks and deformation. 2. Choke the carb, or plug the exhaust outlet, and briefly turn the motor over with a starter. you should see fuel drawn imediately into the carb. A third check is for tightness of the head, backplate, carb, etc. The engine should be securely fastened to flat and parallel mount beams, and turn over easily with the head removed. Finally, with the head on, the cylinder should retain a compression seal for at least a few seconds when the piston is brought up to TDC.