How to test a motor?
You can not properly test a motor without the manufacturer's specifications. Even if you have the specifications, it is difficult to be sure that a motor is working properly without the ability to drive a calibrated load and determine the motor's internal operating temperature.
With a multimeter, you can check the armature resistance, but the proper value is probably so low that the meter will not be able to measure it accurately. For a very small DC motor, you can check the idle current, but you must have a rating from the manufacturer to know the proper value. Note that the initial inrush current can damage the meter unless the motor is very small. It is possible that the meter has a high enough DC current range to check the normal running current, but you would need to operate the normal load, know that the load bearings are not faulty, and know what is the proper current reading.