Hi Ben,
You can't see the motor windings, any loose magnets or possibly any worn reduction gearing unless you press out the shaft from the hub body but you can check for any burning smell.
Check to see if there is a positive pressure on the carbon brushes by carefully easing them away from the commutator into the brush holder they should snap back into place under spring pressure when released.
Lightly clean the commutator with a kitchen scourer pad to remove any rust.
Check with a multimeter that there is continuity across the 2 pins of the motor cable.
If it all checks out ok, rebuild and retry, if it doesn't the work the fault might lie elsewhere.