Here's what you need to do to test (from an article I did for jRations issue 8):
"There are two simple tests to ensure the manifold heater is working properly. These tests must be performed when the engine is cold:
1. Pull the plug from the oil pressure switch and put a jumper across the two contacts in the plug. The jumper is to simulate oil pressure. Turn the ignition switch to the on position and put your hand on the bottom of the intake manifold under the carburetor. You’ll feel a round plate about 5 inches in diameter. Within a minute or two, this plate should get warm. If it does not, either the heater is bad or the temperature switch is bad. You can jumper the temperature switch and try again to eliminate that as a possible
cause. The temperature is switch towards the rear of the intake manifold, near the #6 cylinder.
2. After performing the first test and verifying that the heater does in fact warm up, reconnect the plug to the oil pressure switch. Unplug the connector feeding power to the heater and connect a voltmeter to the connector on the vehicle harness side. Turn the ignition switch to the on position. You should register no voltage at the connector. Start the engine and you should see voltage at the connector, this verifies that the oil pressure switch is operating properly. If you keep the voltmeter connected until the engine warms up, you should see the voltage switch off at about 160 degrees.