What's the best way to find a dead cylinder?

Dale Davies said
Dirt and/or salt on the hose lets the spark track to metal. Humidity in the air is absorbed by the dirt/salt to allow the electron transfer. Eventually it will burn a carbon track into the rubber. A bit hard to see if you are not looking for it.

Check it out at night or in a dark garage with the lights out. A squirt bottle misting the wires with water will help out.