Hook one side of the pot to 5V, the other to ground, and connect you multimeter on voltage setting across the centre wiper pin and ground. Turn the rotor on the pot whilst you monitor the voltage, one end it will be 0 (or close to it) ant the other end it will be 5V (or close to it). A linear should read close to 2.5 volts at the centre position and give a smooth transition from one to the other as you turn the knob. A logarithmic will have 'most of it' at one end of the rotation and 2.5 volts will be close to one end of the scale. Most pots are linear.......
Jumper leads with croc clips at either end make life a lot easier when you're doing this. 