With engine running in mis-firing mode, simply remove and replace one plug lead at a time. Each one that is healthy will make the engine run worse when removed. The faulty one will make no difference to the missing sound.
You may want to don a pair of rubber gloves to avoid any shocks in case of poor insulation.
This test would simply tell you that the fault is in either that particular plug or plug lead and rules out things like condenser, points, coil or fuelling issues, etc.
