Condensation can be a motor killer. Rather than discover condensation forensically, prevent the motor death by actively preventing condensation.
Improper storage of spare motors is one contributor to condensation problems. Store spare motors in a clean, dry, temperature-regulated place. Does your stockroom have a humidity gauge? What about a dehumidifier?
Another contributor is environmental moisture. For example, the motor may be protected from water splash but if it’s still in a place that even occasionally gets damp, that’s a problem you must address. In addition to ensuring the area has adequate drainage, you may need to supply forced air venting (from a dry supply) to the motor or use some other method to help remove moisture introduced from the environment.
Thermal cycling is yet another contributor, and the effect of it is to introduce water into the motor. Motors in outdoor applications are particularly at risk for this. A commonly used preventive measure is to specify a motor with a heater. In an unoccupied outbuilding, such as a fire pump house, a common solution is to heat the building.