DC motors can be easily tested in a number of ways. Many times a motor is replaced only to find another problem exists so be sure to test the motor if you think it is a problem.
Methods to test a motor:
Cordless drill battery- get a fully-charged, 18 volt (preferred) cordless drill battery and on most treadmills connect the red and black motor leads to the battery (some have different colors but if you have two wires and get it hooked up backwards, the motor will just run backwards and won't be harmed). If good, the motor should operate at about 1 MPH.
Car battery- these are 12VDC high amp batteries so the motor will turn slowly but you can walk on the belt. Always be careful with battery acid on these things so we don't recommend bringing them in the house?take the treadmill to the battery.
Lantern battery- a fully charged lantern battery should operate your motor at a slow speed but lacks the amperage to have someone walk on the belt.
Use an outside DC converter with a variable speed input. The most complex but also the best way to test a motor. If you have a way to do this, it is the preferred method and how we do it in the field on local service calls.
For other motor diagnoses, see the Loses Power When I Step on the Belt test topic.