The electrical test light is one of the cheapest, simplest, and most versatile tools. It is simply a light bulb with two electrical connections. Connect the alligator clip to any convenient grounding point, then probe some electrical contact point with the tip. If there is electrical power present it will light up. That's all it does, but it can be your first tool of choice for diagnosing electrical problems.

Below, my current favorite, because it's small (picture full scale),
Dorman part number 86612 - found at O'Reilly Auto Parts.