We specialize in testing and aligning feedbacks and we don't have much expertise in other types of motor testing. Below are a few simple suggestions.
Most motors will have a torque rating on the nameplate, and a corresponding stall current rating right next to it. The idea here is fairly simple, you lock the motor with the rated current and then use a torque wrench on the motor shaft to make sure it is holding the rated torque. In the example below the motor would be locked with 4.85 amps and the motor shaft should hold up to 1.58 Nm of torque.
Another test that can check magnet strength is back driving the motor and recording the voltage coming from the windings. This value is called a voltage constant, often labeled as Ke or BEMF and usually rated at 1,000rpm spin speed. Some motors print this on the nameplate but most do not. However, most manufacturers will list it in the technical data of their motor manuals. You simply spin up the motor under test with another motor at 1,000rpm and read the AC voltage generated from the motor windings (U reference to V phase). Weak magnets, which result in weak holding torque, may have Ke values that vary greatly from the technical rating.