One of the symptoms of a bad lawn mower ignition coil appears when you are mowing the lawn and the mower gets hot and literally shuts off. The mower must cool down over a period of time and will start and run for awhile until it gets hot again, at which time it will shut off again. The only real way to determine if it is a bad ignition coil and not another problem is to test the coil itself.
Pull the spark plug boot off of the spark plug, and replace it with the boot of the in-line spark tester.
Plug the free end of the tester into the original spark plug boot. Push it in firmly to make sure there is a solid connection.
Start the mower when it is cold, and look through the window of the tester. Observe that there is a normal spark that allows the mower to run.
Replace the tester with the original spark plug boot and start up the engine. If possible, mow some grass to bring the mower up to operating temperature so that it shuts off again.
Replace the original wire with the in-line spark tester, and attempt to start the motor. Watch for the same intensity of a spark that you saw when the mower was cold, If there is little or no spark, the ignition coil is bad and needs replacement. If the spark is good but the mower refuses to start, it's another problem unrelated to the ignition coil.