A very good question which requires participation from maximum maintenance engineers disclosing the various techniques used by them for predictive maintenance of motors.
Following online techniques are used in our plant for determining the motor condition
1.Motor current measurement and logging every
2.Motor Temperature measurement and logging
every 15 days
The above logged data are compared with the previous ones and also with the rated specs.If found beyond limits,motor is taken for overhauling/replacement on available opportunity.