You could certainly attach a piezo contact microphone to the case, run that trough a 100Hz filter, peak detector and comparator.
But a more reliable method is clamping a current transformer around one of the wires and sensing the current. It always draws an abnormally large current when stalled.
While you are at it you might as well also connect that sensor to a relay that disconnects power from the motor. I'm guessing you are asking this because you have a motor that is likely to stall, and in that case it would be wise to have a automatic system that kills power to it since leaving it stalled for any significant length of time is sure to destroy its windings to charcoal. But a carefully chosen value slow blow circuit breaker should do the job by tripping when experiencing a stall current for too long. A circuit breaker is much cheaper than a brand new motor