A potentiometer can be essentially thought of as two resistors connected in series, in which you can vary the resistance of each; so if you have a 100k ohms potentiometer, and you rotate it 1/4 of the way, when you measured each resistance, you would find one side is ~ 25k ohms, and the other ~75k ohms (I use ~ to mean "approximately"; due to losses and other reasons, you may or may not be able to add both amounts together to equal the whole amount; this is called "tolerance", and all electronic components, including potentiometers, have a measurable tolerance amount - the smaller that amount, the more accurate a component will be - note, though, that it can -never- be perfect; ie, zero percentage of tolerance).
So - a potentiometer typically has three lugs, or spots on it where wires can be connected. You have the two ends of the whole resistance, which, if you positioned the potentiometer so that the shaft pointed vertical, and the lugs pointed toward you - would be the left-most and the right-most lugs. The center lug connects to the "wiper", which is nothing more than an electrical contact which slides along the resistance path (rated for the whole amount of the potentiometer; for our example, 100k ohms) connected between the other two lugs.
If the value of the potentiometer is known, then you set your multimeter to the resistance reading level that is higher than the whole resistance of the potentiometer; for our example (and with the simple multimeter I own), for the 100k ohms potentiometer, I would set the meter to "200k", because 100k is less than this amount, but greater than the next setting down of 20k.
If you don't know what the resistance is, then you set your multimeter to the highest setting; if you measure and are able to read something, reduce the setting. Continue to measure and reduce, until you measure and the multimeter gives you an "error" reading; when this occurs, adjust the meter to the next highest setting, and measure a final time.
For measuring a potentiometer's total amount then, you would measure between the left-most and right-most lugs (if it is oriented as described previously). Then, to measure the current setting, measure between the center lug and the left-most lug; this is one resistance level. Then measure between the center lug and the right-most lug; this is the second resistance level. Adding these two amounts together should equal (or be very close, depending on the tolerance percentage level) the original whole measured amount.
Note: This is only truly valid with the potentiometer out-of-circuit; that is, not connected to any other portion of the circuit. Because circuits are, well, connected networks of varying levels of capacitance and resistance (at a basic level), measuring a potentiometer (or any other component, really) in a connected circuit may not give you a reading you are expecting, depending on a number of variables and the design/construction of the circuit.