There are a number of ways depending what tools you have.
1] Take the bulb out and test it using the tractors battery. One wire to the brass case of the base and one wire to the center post on the base. The wires go to the battery posts.
2] If you have a volt meter, set the scale to 20 volts and connect the Red led to the wire/buss that feeds the plugs and the Black led to frame ground. You should see about 12 v with the plugs activated.