I’ll probably write an Arduino sketch and output a pulse count via serial. Have it run the motor for a few seconds and that should get me an estimate if its out to lunch or not.
That’s an approach from the software angle 
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The firmware treats every transition, rising and falling, on each of the two quadrature channels for a motor as a pulse. Because they are quadrature and 90 degrees out of phase, you can count one channel and multiply by two.