As you say, you can probabally safely assume that the Red & Blk are Power & common. Apply power and apply a voltmeter to common and the O,G & W in turn and as slow as you can, turn the shaft, you should see a transition from common to + on each, the exception will be the marker pulse which will only occur once per rev. so it is hard to catch.
If you do not see any results on any, try a 1k resistor for each to +, if you now see results, it is probabally an open collector output.
If it is a brushless motor however, this may be the hall commutation pulses, and if so you will get large resolution pulses out, all three the same and the amount will depend on the number of poles.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.