The first thing you need to get is a multimeter this will help you test it. There might be a function on the meter that will give you the amount of voltage in that particular area. Then you have to put foil tape on all of the inside of the guitar to test it. You need to get two wires from the meter and then put them both on the foil tape on the guitar. If you don't hear a beep then you add more tape in that area where there is no activity. You can also test this on the ground.