One of the easy tests you can do to see if you are getting good spark is to remove the lead from the plug and hold it close to a grounded bit of metal on the engine block to see if you get visible spark between the plug lead and the ground. Has to be done with the engine running. I guess this tool would allow you to pull off each lead in turn, hold it safely while testing, and then replace it. I've done this test before, and I can tell you that it's tricky to manage without getting a couple of jolts!
You can test for a misfire or no spark. With the engine running, if you pull the plug wire off and idle speed decreases then that cylinder is working fine. If the idle speed stays the same with the plug wire removed, then there's a problem with one of the components in that cylinder
