DIY: Make a Test Light (Video)
 

Video tutorial on how to make a cheap and easy temporary test light. If you have all the materials laying around, you should even be able to make this for free! This test light can be used to determine if you are receiving power at a certain area. This is a 12V application. The test light is unable to determine how much power an area is getting, but you will see if there is a drop in voltage if the light is dime or not as bright when it is connected directly to the battery.

Tools/Supplies Needed:
-wire strippers
-electrical tape
-12V light bulb (tail light, brake light, marker light, reverse light, etc)
-2 12" lengths of wire or 1 double molded 12" length of wire

Procedure:
-find an appropriate light bulb for the application, mine came from the tail light
-cut a length of wire to the appropriate size using the wire strippers
-strip the casing off the wire on each end, 1/8" to 1/4" in length of bare wire exposed
-using the electrical tape, secure on wire to the side of the light bulb casing which is the negative contact
-using the electrical tape again, secure the other wire to the bottom contact of the bulb which is the positive
-add extra electrical tape if needed
-test light on battery to ensure it works

I know I don't have a lot of posts. But please don't do this. Get a proper DMM. They aren't expensive. You can still get a dim light and it would simply be because that isn't the highest resistance in the circuit. There are plenty of reasons to not use this. A DMM will give you a much better idea of voltage drops in the circuit.
Sidenote: It HAS to be digital. Analog meters will damage computers.