One of the best ways to check electric motor operation, is to put an ammeter in line with it.
Of course it helps to know what the current draw of a know good motor looks like.
Other clues are temperature of the motor case.
Taking it apart for visual inspection, looking at the commutator and brushes, and measuring the field coil and
armature windings, looking at the internal insulation for overheating or obvious breakdown.
These steps will isolate the faults in any electric motor.
But, if there is a mechanical problem that results in binding, that can cause excessive current draw and overheating.
Do you blow fuses, or does the motor get hot ? Those are important questions to ask first.
