To test a pre engaged starter motor as you have illustrated proceed as follows.
Securely clamp the starter motor, in a vice preferably, and attach a heavy lead from a 12v battery negative terminal to the motor casing. Again using a heavy lead attach the battery positive to the starter solenoid vacent large bolt on terminal. Using a light jumper from this terminal to the solenoid initiate terminal will initiate the starter and if all is well the the drive pinion gear will move forward and the motor will rotate.
The drive pinion gear is normally quite stiff to rotate by hand on the shaft so do not consider this as a problem.
In the interest of your safety please ensure the motor is secure before you try this test as the rotational torque is high.
For satisfactory operation the motor/solenoid action should be both smooth and free running with no untoward knocks or clunks.