zero speed sensor?

I wish to sense the speed of a gear. Should I be using a zero speed sensor? What's the difference between zero speed sensors and a normal inductive sensor?

A zero speed sensor is an inteligent piece of kit that will have a prox type sensor with some electronics to check for low or zero pulses from the actuator on the shaft/conveyor, these usually have trimmers to select the "speed or lack of pulses" that the unit will turn on the output i.e. if set to 10 ppm then if the time between two pulses was less than the expected time for 10ppm (pulses per minute).
these are used generally to detect jams in elevators or conveyors & stop the drive to prevent burnout or other major damage.
if you use a sensor then you will need some logic to determine if the shaft/conveyor is moving & what speed you wish to alarm at or even use as some crude form of speed indication.

Thanks for the reply!

http://www.electro-sensors.com/ I have used these and they work well for your purposes. Give them a look!