Assuming you have AC to the cord try this in the dark. Quickly disconnect the block heater cord from the extension cord or electrical receptacle. You should see an electrical arc (flash).
Ideally you should use a ohmm meter. If you have a piece of hose like a garden hose you can place it very near the block heater and listen for sounds with your ear. It should give off a gurgling sound.
If your block heater is shot you might try a lower rad hose heater. They are somewhat easier to install, but it depends on your cooling system. TSC stores sell them. My Saab had one.
