"Having 12 volts at the brushes is one thing, having continuity through the motor winding's it a totally different thing. Sounds like the motor winding's are open. Check that you have continuity through the winding's on the motor. JMHO "
Are you talking about wiring on the rotor?
What is the proper way to check it?
Yes the rotor windings create the magnetic field to work against the magnets surrounding the rotor to spin. So make sure the rotor windings are good and that 12 voltage is getting to those windings. If the 12 volts is getting to the windings and the windings are good but motor doesn't run, you have some bad magnets on the casing. Which really doesn't sound like a plausiable condition, but there is no other possibility.
I am thinking that the rotor commutator needs cleaned and polished so that the voltage is transferred to the rotor windings myself. WIth the rotor having so many windings all around the armiture, the only way the motor wouldn' t run is for all those windings to be burnt open. Again that really doesn't seem like a plausible condition, but there really isn't many other options IF you do have 12 volts at the brushes. But not being there and testing things myself, these are only wild guesses.
Remember you have to have two brushes make a complete circuit with the copper finger on the rotor commutator to work. Reading 12 volts from one brush to the battery ground doesn't mean you have everything correct. The second brush has to also be working to complte the circuit. So make sure you are reading via at last two opposing brushes for verification. JMHO