A digital multimeter is a useful tool that simultaneously acts as a voltmeter, ammeter and ohmmeter. As with any type of equipment, the more money you spend on a multimeter, the more functions it has have and the better it works. When testing or replacing electronics on a guitar, a digital multimeter can tell you several important things, specifically whether the pickups and potentiometers are working correctly. From the information garnered from the multimeter reading, guitarists can work to diagnose and fix any electronic problems their guitar is experiencing.
Isolate the electronic piece you want to test. You likely access to the guitar's cavity to safely unsolder the wires attached to the pickup or the potentiometer on which you are going to use the multimeter.
Attach the black lead to the common terminal on the multimeter and the red lead to the V-ohms-diode terminal. On some multimeters, the black lead plugs into the outlet, and the red lead goes into the "+" outlet. Plug in the multimeter once these pieces are attached properly.
Choose the desired multimeter setting. For guitar applications, measure resistance, ohms.
Touch the tip of the black multimeter lead to the ground wire of the pickup you want to test. Touch the red wire to the hot wire. If testing a potentiometer, place the pot on its side, so that you are facing the back of the pot, with the wire connectors facing up. Touch the black lead to the connector on the far left, and then touch the red lead to the connector on the far right. Allow a few minutes for the reading on the multimeter to stabilise. Write down the reading.
Not all types of pickups are meant to run on the same amount of ohms. Read the pickup's instruction manual, many of which can be accessed online, to find proper resistance figure. Most guitar potentiometers properly run at either 250k or 500k.