Measure on the hot DC lead (usually red) going from the controller to the drive motor. If you have a simple multi-meter with a DC amp test setting the best way to test is to put the meter inline on the red motor lead with alligator clips. If you have a more expensive clamp meter, clamp the meter around the red motor wire. Run the machine at 2.5 MPH without a person on the belt. If the motor and drive system are healthy, you should have a reading in the 2-3 amp range. Then walk on the treadmill at the same speed. With a person of average size (around 200 lbs.) you should have a reading in the 6-8 amp range if the belt is good. The readings will fluctuate more with a load but you are looking for an average reading but you shouldn't see spikes above 10 amps. If the amp readings are high without a load, you could have a problem with high resistance in the motor, a bad bearing, or over-tightened belts. Regular readings without a load but high readings with a load indicate a worn belt and/or deck.