Out of circuit with a conventional analog meter. Set it to the resistance scale closest to what the pot's value is.
Connect your meter leads to the two outside terminals first. Measure the value of the pot. If it's a 10k pot then it should read around 10k. If it does not show any reading the pot is open (bad).
Now connect the meter leads to the center terminal and one of the outside terminals. Turn the pot's knob and watch the meter needle swing. It should be smooth as it moves from one extreme to the other. If the needle jerks/bounces around, the pot is "scratchy" (dirty inside). If the needle drops at a certain point when turning the knob there's probably a hairline break/crack in the carbon suface inside the pot. The pot needs to be replaced if that's the case. Reverse the outside terminal meter lead from the existing terminal to the other outside terminal and repeat the same test.