Test the motor windings for resistance, and perform an insulation resistance (megger) test from each motor phase to earth.
Black - Primary Winding
Red - Secondary Winding
Green - Common

Resistance is lower for higher powered motors, usually a few ohms for the primary winding and about 3 times higher for the secondary winding. A zero or very high resistance is a fail.

Insulation resistance should exceed 3 megohms.