See Figures 1 and 2
Spark plug wires are those which carry electricity between the center tower of the distributor cap and the center tower of the coil, and those that carry electricity between the distributor cap and the spark plugs.
These may be tested visually by gently bending them and inspecting the bends for signs of cracking. If cracks are found, replace the wires. It's a good idea to replace the wires in sets, rather than individually.
The wires should also be tested for resistance with an ohmmeter. Resistance should be 16K omega per meter (1000mm).