BertKanne says
With the engine running in a dark environment and while wearing insulated gloves, open the hood of the vehicle and look for signs of any sparks coming from the area of plug wires (or anywhere else for that matter). Move the wires around and note any changes in how the engine runs and any sparking. If either occur, change the plug wires. A good set of replacement plug wires should last the life of the engine. Heat, humidity, time and the quality of the plug wires can affect their life. If the vehicle has a distributor cap and rotor, change them at the same time. Original parts should normally last 75-100k miles.
If the spark plugs themselves haven't been change in the last 75-100k miles (for modern vehicles) it's a good idea to change them as well. Be sure to use the correct plug for your application, correctly gaped and use anti-seize compound on the plug threads. Be very careful not to cross thread spark plugs. Spark plugs must be torqued to the correct ft lbs, which is very important on vehicles with aluminum cylinder heads.
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