how to tell variable reluctance sensor

Other Hardware Sensors
4

sammypati:
since I posted the above message a friend of mine did some digging and found the Post where I must have gotten the diagram from all time ago.

I had the diagram wrong anyway so if your interested you should look at this post http://forum.arduino.cc/index.php/topic,157697.0.html

I also attached a correct version of the diagram
The thing I didn't know about VR sensors is that they are passive, i.e. they done take any voltage in - they produce a small voltage , I think its a sine wave +/- and the LM393 is used to amplify and turn into 0 or 1's

However , the question still stands, is there an easy way of determining if the sensor is a VR one?
In the above Link , user 'DC42' seems to have know how to tell but never actually mentions how .

The VR probes I've used at the refinery I worked on were simple 2 wire devices that feed a comparator to give a digital square wave output at a frequency proportional to the speed and number of teeth that the probe was sensing. I believe the internal construction is a magnet with a coil around it, and as a gear tooth passes the sensor face it changes the field of coil causing a change in the sensor output voltage. The sensor output voltage looks like a quai-sinewave and it's voltage increases with increasing gear speed, however once the voltage is above a certain minimum value the comparator doesn't care as it's just switching at zero crossings of the probe's output voltage.