Background
A potentiometer is a variable resistor. There are three leads. A constant value resistor connect two of the pins. The resistance between the 3rd (wiper) pin and each of the others is controlled by the turning the knob. With a linear potentiometer, if the knob is half way between completely clockwise and completely counter clockwise, the resistance between the wiper and the other two pins should be equal. If it is algorithmic, they will be different.

Prerequisites

• Ohm Meter (or volt meter - instructions for this coming soon)
• Potentiometer disconnected from original circuit

Instructions

1. Measure the resistance between all three combinations of the three different pins (1st and 2nd, 2nd and 3rd, 3rd and 1st)
2. Find the combination whose resistance doesn't change as you turn the knob. These are the constant pins.
3. Locate the other pin. This is the wiper pin
4. Turn the knob all of the way clockwise
5. Note the position of the knob
6. Turn the knob all of the way counter clockwise
7. Note the position of the knob
8. Turn the knob half way between the previous two positions
9. Measure the resistance between the wiper pin and each of the constant pins

Results
If in the last step of the instructions, the two measured resistance were the same, you have a linear potentiometer and it should work well for what you need it for. Otherwise, you should probably try to find a different one (although an algorithmic potentiometer should work in a pinch)