A cheap multimeter might show any big resistances:
http://www.8886.co.uk/ref/ohm_meter.htm
You set it to the horse shoe symbol "omega" in the most sensitive setting and use the black and red wires. Connect the leads so that it measures resistance. Test the setup by touching the two leads together and it should read "0" resistance.
Though to measure very fine resistances you use an ammeter along with your voltmeter.
http://en.wikipedia.org/wiki/Ammeter
If I remember, I think it would be beneficial to everyone to know the what parts of the car's electrical system benefit the most from adding an auxillary grounding wire. Measured with the ammeter/voltmeter method.
What's interesting is an ammeter works on the same principal as an inductive loop sensor. An inductive loop sensor is those circles or lines you see at stoplight intersections that tell the stop light that a car is waiting at the red light. These sensors are basically wires sawcut in the asphalt forming Loops. When a large piece of metal/iron goes inside the loop (Such as your chassis/engine), then the electrical signal changes. This tells the light that a car is there.
The sensors function by the metal in cars, not by the weight which is why motorcycles sometimes have hard times setting off the sensors.
Ok that's enough science for the day...