back.
Check for suspect smell of burning or quick heating up which may be your problem.
Next step is taking it all apart, and take good note of it where it came from, can use a digital camera and take piccies as you go, be prepared for dirty hands from carbon dust, and old grease.
Normally a starter motor has a rating on it expressed in HP or kW. and direction of rotation, embossed in the stator housing.
Different types of starters are in use too.
Bendix: the pinion gear engages itself into the flywheel ring gear, by the slow inertia when the starter current flows. (used on older English cars, Lucas)
Forced engagement: the solenoid is mounted above the starter motor and assists in meshing the pinion prior to closing the contacts and allowing full current to flow. ( Petrol and diesel engines )
Shifting armature: the whole armature shifts and engages the pinion in one or two different stages. ( diesel engines, Bosch )
Reduction gear starter: Used over the last 20 years or so. A relative small high speed starter motor drives the pinion via an intermediate gearwheel, and has very good torque for starting and less demand on the battery. ( used on mostly Japanese cars and trucks )
Dyna starter: Comination of alternator/starter used on motor bikes.
Testing a starter motor is done on a testrig on which the starter is securely clamped, the correct meshing gear is engaged and a brake can be applied to increase the load on the starter to blocked armature condition.
Volt, Ampère and Tachometer are on the test rig to check current flowing, voltage across terminal and speed of the pinion.
The tests have to be done reasonably quick to avoid excessive heating of the windings affecting the test results.
Also good and clean connections are a must !
The resistance in these circuits are between 0.1 and 0.01 Ohms.
Ok that is my $0.05 worth.
Regards, Raymond