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).
However, be advised that you are running a combustible mixture through that cylinder, but without a spark to ignite it. So the unburned mixture ends up in the exhaust and eventually in the atmosphere. You might want to make sure this doesn't do any harm, and doesn't meet another source of ignition somewhere.