Woody,
IIRC, I asked this same question to my friend that owns a motor repair shop. Again, IIRC, the resistance value of the windings, measured with an ohmmeter would be about the only way to tell, and I have no clue what they would be.
Having said that, my thought would be to 'listen' to the speed of the motor. I think a 6 volt motor connected to 12 volts will sound 'too fast' even to the untrained ear. If you hook your motor up to 12 volts and it sounds reasonable, I'll bet it's a 12 volt motor.
Your observation about wire insulation could be right on the money. A true vintage motor would probably have the brown cloth covering whereas a more modern 12 volt motor might have true plastic insulation, but this again is just a guess.
Stuart
