I've done a lot of motor and fan sensing. There are about 19 ways to do this, so more data is required to get to a good solution for your application.
Shaft speed in rpm or rps
Shaft diameter
Environment (wet, humid, dry, dusty, medical, food processing, etc.)
Output signal (transistor switch to GND, opto isolator, email message, etc.)
Power available to run the detector (AC/DC, volts, amps)
Distance the signal has to travel
Minimum detection time - how soon do you have to know that the shaft is moving (1 rev, 0.1 rev, 5 seconds, etc.)
Minimum release time - how soon do you have to know that the shaft is stopped
ak