To find out what spark plug is duff, I've heard that engineers start and run the engine on the "rough" magneto for a minute or so, then shut down. The cylinder that's cold is the one with the duff spark plug (or HT lead, or whatever).
This is also a good idea, but use the exhaust temp rather than the cylinder.. You don't need to run the engine for more than 30 secs as the exhaust heats up quickly. you can tell by touching them which one is cold or cooler.