Get a spay bottle of water that makes a mist. Then either find a very dark place or wait until it is dark outside. Spray the cap and wires down with the spray bottle and start the car. You will see a spark show from whatever spark plug wire or other ignition part that is leaking electricity.
WD40 is an excellent treatment for wet spark plug wires and distributor cap. However, it is not a long term fix for such problems.
In the old days you could buy a product called Wire Dryer that worked well for me (on the insides of caps, etc). I don't know that it is available anymore.
The mist test that JRichker posted will probably reveal lots of pixie dust. Be careful of moving engine parts, etc.
What about one of the rubber distributor covers (after you find the problem).I have mine still on the engine and have never had moisture issues with the distributor.