To test a lawn mower coil, you will need a multimeter. First, disconnect the spark plug wire from the spark plug. Next, attach the ground lead of the multimeter to the engine block. Then, touch the red lead of the multimeter to the terminal on the coil where the spark plug wire was attached. If the reading on the multimeter is between 0.5 and 1.5 ohms, then the coil is functioning properly.