<3.0L Engine>

Remove the spark plug and connect to the spark plug cable.
Ground the spark plug outer electrode (body), and crank the engine.
Check that there is an electrical discharge between the electrodes at this time.

Fig 1: Testing Spark Plug - 3.0L Engine
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.

Fig 2: Locating Spark Plug Defective Points - 3.0L Engine
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.