Insulation weakens with age. Period. Stop.
Some insulation is so robust that it can hang tough almost forever: MI.
Weakening insulation within motors is tested by 'megging it' with -- a Megger.
Google and explore.
A known high static voltage is imposed upon the windings, one by one and their RC curves are monitored.
This is how the very beginning of corona discharge is spotted.
Leakage at high voltage precedes leakage/ shorting at operating voltage.
So, there is a way to track a weakening motor before it goes all together.
A regular DMM is not going to spot anything until it's toast.
A GFCI/ RCD will trip before the windings are a fuses mass of metal. Even so, the cure is the same: you need to pull the motor and rewind it or replace it.
These days, motor replacement is almost always preferred.
Being machine built to the latest standards makes them both cheaper than a rewind but cheaper to run, too.
Which is why all of the motor rewind shops of old around here have gone belly up.
