Sometimes plug wires will fail in an obvious way and produce a wonderful light show when you open the hood at night. Usually though, the wires are breaking down internally. Sometimes the conductor will break and force an unintended spark gap halfway to the spark plug. That puts a higher load on the secondary of the ignition circuit. Symptoms include breaking up under heavy load or high RPM and increased fuel consumption. If the spark is viewed on an o'scope, the broken or high impedance wire will produce a spark peak half-again as high as his neighbors. Actual misfires will be right off the scale.
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Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL