Im looking to do the same thing, I have a pair of Honeywell motors with some encoders on the back. I cant figure out how to get any sort of reading out of the encoders though. I took one apart and got the pin connections. It looks like there are only 2 measurements that are taken off of the encoder disk and then it is put through an inverter to give 4 outputs.
I'm able to see some waveforms, but they dont seem to change much, even with a very slight turn of the motor shaft I get many oscillations (probably 20 or more) in the output .
To answer your question, you can use the encoder to measure how far the motors travel.