If they are with anti-friction bearings, open the end bells/end shields, remove the old grease, check bearings and re-grease.
Measure IR and PI values of the windings to start with.
If PI>2, do an ac Hipot at 1.5 times the rated voltage.
If possible, do a surge comparison test.
Do a uncoupled, spin test with reduced or rated voltage.
* The shin is the device to find your furnitures in the dark *