Start the engine at night, and with very little ambient lighting, see if there is any sign of "flash over" on the leads (pale blue glow or flashes to earth/body/engine).

Improve on this, make it a "torture test" by taking a spray bottle of water and lightly misting the plug wires, distributor, etc. while the engine is running. You should not get any arcing or misfiring when you do this. If you do, replace plug wires, distributor cap and rotor (assuming the car is equipped with a distributor and not some fancy multiple coil system driven by a crankshaft sensor.)

tanstaafl.