I saw a neat way to check if all of the plugs on a motor are firing or not. Instead of pulling the plugs and trying to fumble around grounding them while cranking it over, hook up a timing light and move the clamp from plug wire to plug wire while holding the trigger down and looking at the light. the aligator clamps to power the timing light can be hooked to the engine for ground, and one side of the starter slave solenoid. Everytime the plug fires the light should flash. this method is great for on lake testing because theres no chance of dropping tools or spark plugs into the lake.

Won't the timing light flash even if the spark plug doesn't fire?

Yes you would be able to check the coils/power pack/ wires to tell if you are getting Fire Too The Plug... but would not be able to tell if the plug is firing, fouled or has a burnt electrode.

good point.

If you are running the motor out of water with ear muff hook up which you would about have to be to hook up timing light, you can run the motor for a couple of minutes, shut it down and just feel the plugs, misfires will be cold. s

Your gonna have to pull the spark plug and hold plug with wire connected and look to see if you are getting a fire across the gap. Like what was said above, the timming gun is only gonna tell if you have juice running to the plug on time and correctly.