A potentiometer is a cheap electric controller used for everything from dimmer lights to electric guitars. A potentiometer is a variable resistor--a device used to resist the flow of electric current. As you turn the potentiometer, it increases the resistance, lowering the lights or the volume on the guitar.
Find out the rating of your potentiometer. The total resistance in ohms should be written on the side or bottom.
Set your ohmmeter to a setting higher than the total resistance of the potentiometer. For example, if your potentiometer is rated at 1,000 ohms, set your ohmmeter to 10,000 ohms.
Look at your potentiometer. There should be three tabs sticking out of it. Two are called the "ends" and the third is called the "wiper." Usually, the two ends are next to each other, and the wiper is in a different spot.
Put the probes of your ohmmeter on the two ends. It should read within a few ohms of the rated resistance of your potentiometer. If you get a different reading, one of your probes is on the wiper. Try different combinations of two probes until you get one that gives you the right reading. You now have the probes on the ends.
Turn the controller from one side all the way to the other while keeping the probes in contact with the ends. The resistance should either not change at all or change only very slightly from one side to the other.
Take one of the probes off one of the ends and put it on the wiper. Slowly turn the knob all the way from one end to the other while watching the multimeter. At one end, it should have a resistance of only a few ohms. At the other end, it should have the maximum resistance. It should slowly and continually increase as you turn the knob with no sudden jumps.
If any of the readings on your potentiometer are wrong, replace it.