Do this at night, as dark as possible.

Find a very, very dark corner of town -- no streetlights or porch lights, no moon.

Turn off your interior light. (Be sure it will NOT illuminate when you open the door.)

Turn off your head lights.

Use NO lights, allow you eyes to "dark adapt" for about 10 minutes.

If you have an underhood light, be sure it's switched off before you begin.

With the engine idling, open the hood (use NO flashlight), and watch the ignition wires, distributor cap, plugs. Any weak components will display a very dim, weak blue corona discharge and/or arcing, sorta like St. Elmo's Fire.

Often this is difficult to see, but your eyes are more sensitive to very dim light if you look slightly aside from the object. In other words, you can more easily see very dim lights in your peripheral vision than direct vision. (This is the rods and cones thing.)

Reach under the hood and nudge the throttle -- this will sometimes cause breakdown of weak ignition wires and you'll see arcing or corona discharge.

If you see this type of discharge, replace the components.

You can sometimes encourage it to happen by misting the ignition components with a spray (Windex) bottle; use water.

This does NOT cause good parts to fail. Instead, it reveals marginal or bad ignition components that will otherwise cause you grief on a wet, humid day.

Your engine should be able to withstand a bath while idling and not miss a beat.
--
Don Foster (near Cape Cod, MA)