Have you ever had your electric guitar cut out on your before. Maybe you bumped it up against something while you were playing it and the signal just died. This happens to all kinds of guitarists. Usually either a broken solder joint or a dead battery causes this problem. The real problem isn't that your guitar or a customer's guitar is shorting out. The real problem is finding the short. You can use a multimeter gauge to help find where the electrical current is lost.
A multimeter gauges measures the flow of electrical current. You take the positive and negative wires and hold them against the component that you are testing. The multimeter gauge measures the AC/DC voltages, amperage, ohms of resistance, and indicates circuit continuity. It can be used to measure pots, pickups, amp circuits, and other electrical components. You can also use a multimeter gauge to test whether your 9-volt battery works before you take it on stage with you.
If you don't do much electronic work, you could probably pass on this. There is no need to get a multimeter if you are never going to use. On the other hand, if you are doing any custom guitar building or repair work, you should probably have a multimeter gauge in your guitar repair bench. It will help you with all of your electronic measuring. If you would like more information about wiring electric guitars, please see my guitar electronics repair article.