How do I test the value of an unmarked pot?
Digital multimeter, an ohmmeter of any kind.
-Colin
The circuit it's in will generally affect the vaule you read so the best way is to desolder two of the wires (the wiper and one of the ends) and then place your meter across the ends to get the value.
Keep in mind that pots are generally very loose tolerance so a 500K pot might read anywhere between 400K and 600K in the real world.
Take care,
-Peter