not real familiar with stepper motors so I was curios when to give up on a motor.
The short story is I have two six wire steppers. they are nema23 Japan servo steppers. I got them with a compact 5 cnc I bought off ebay. neither of them has the wire harness so Im just connecting to the pin connectors on the motor itself so there are no colored leads to tell wich winding is which. I pretty much figured that out though. got one of them running beutifly smooth. wired the other up the same way and it runs like dirt. sputtering and just behaving in non linear ways. Ive tried changing around the connections but nothing makes this thing run better. so I guess my question is how does a stepper go bad and what does it look like. just want to be able to tell if I should give up on the little guy and order up a new one.

Have you tried swapping motors on the drive that works? If the one works, and the other don't, then the steppermotor must be faulty. Another way to check motors out of circuit is quite simple. With the motor removed and disconnected, short all the windings out together and physically turn the motor shaft. The motion will be resistive but smooth. If you can feel a ripple effect, chances are one of the windings is open circuit somewhere. You can also measure the windings with a multimeter on ohm setting for consistency of resistance between windings. If you can find the leads that belong to the same winding in the motor, shorting them together and turning the shaft will be resistive and you should feel a ripple effect as the rotor passes the poles which are shorted (static torque). Doing this by alternatively shorting windings should have the same load resistance when you rotate the shaft. If you have an oscilliscope, you can spin the rotor shaft with a drilling machine and observe the sinewave on each winding.

Whacko when all else fails!