- Get yourself a multimeter (a cheapy Chinese one from the dollar store is just fine)
- Set its range to Ω (= "Ohm") and start with, lets say, 200k (you won't fry anything if you chose the wrong Ω-range)
- Put the red lead in the V/Ω input and the black lead in the "Common" input
- Turn on the meter
- Test the meter: put the metal contacts of the leads to each other: the meter should show (around) 0 Ω
- Put one lead (doesn't matter which one) on the left pin of the potentiometer and the other on the right pin (the middle pin can be left loose)
- The value shown in the display of your meter is the potentiometer value you're after; it could be off by a few ohms though, so pick a potentiometer that had a round value slightly lower than what you measure
- If your meter shows "OL", chose a higher resistance value on the meter, when you get a very low value (or "I"), you might want to try a lower value
Have questions about FETS, transistors, measurement, power supplies, or anything else electrical?
If the value is half the total then you want a linear pot, if it's not then you want a logarithmic pot. There are such things as reverse log pots but they are not very common.