Typically a 6V motor will have heavier gauge windings, but probably not enough so to tell difference easily. Part numbers will help if you can find reference to them. Coil resistance can also help, but need to have precise multi meter and again have reference material to tell which is which.
My 6v motor in my 52 had to be repaired , probably due to using it when truck was converted to 12v for prolonged times.
Using it now (12v) through a ballast resistor, it runs pretty quiet and blows moderately. If direct (no resistor) its giving a decent hum and blows fairly hard.
Hope this helps. Not sure if I have part number from mine somewhere. Don't remember seeing one when I had it out, but I'll check.
