Here is one way to check to see if a stepper motor is good or not...
(NOTE: If this test passes, the motor is not necessarily a good motor; however, if it does fail, then it is a bad motor and needs to be replaced.)
The idea behind this test is to check the resistance of the phases. The two phases should have the same resistance. And there should be no connection between either of the phases or between either of the phases and ground.
A two-phase stepper motor will have at least four wires:
A+ and A- (which is one phase)
B+ and B- (which is the other phase)
If there are six or eight wires:
A+ centertap (these two centertaps will be pre-connected in a six-wire motor.)
B+ centertap (these two centertaps will be pre-connected in a six-wire motor.)
If there are two centertap wires, connect the A-centertaps together, then connect the B-centertaps together before performing these tests...
Measure the resistance with an ohm-meter between A+ and A-. Note the resistance.
Measure the resistance between B+ and B-. Note the resistance.
Do the two phases measure the same?
Measure from A+ to B+. Is there any connection between them? It should be completely open...infinite resistance.
Measure from A+ to the ground wire if there is one. Then do the same with B+. These should also be completely open.