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 .