Even though a fault in your motor can be found out with the help of a multimeter, still becoming doubly sure really helps. You can test your motor insulation with the help of a megger. If you want to test a 480v motor, then the voltage required would be 1000v with a reading of 2M ohms or more. The ideal mode of testing is phase to phase and phase to ground.
Next you need to conduct a threading balance check by evaluating the three phase currents. Sometimes out of balance voltage might also display unbalance as well, so you will have to be careful with this. Also conduct a rough resistance test to assure that the windings on the motor are the same.
Apart from this you can also run your motor without load, which is also a good way of testing your motor. Even though you may have to do a bit of work by splitting the coupling, this test will really help you to find out if the motor is bad or good, since it is not usual for a bad motor to run off load.
Additionally along with these tests you will also have to test your voltage, contacts contact check, terminal check, etc.