how do I test each pot / slider for defect?

I brought one of my JPs to a tech and he did an onboard test the JP has that shows if there's a problem with any of the knobs / sliders. I think it was something more than I know you can hold Shift and turn a knob and it should show on the screen something like 0 to 127 but I don't think he sat there and like slowly turned each knob and counted to make sure it read all 0 -127. I think there's something you can do that an * would pop up or something? I just want to test a used JP I bought.