long or short cage deraileur? How to tell.

[mihlbach]

I just bought a used 9 speed Shimano XT derailleur on ebay for a 2x9er that I am building. The seller advertised the derailleur as a medium cage (which is what I wanted), but having received it, it looks like a long cage to me. I can't find any information about cage lengths for short, medium, and long cage derailleurs. There is nothing stamped on the derailleur that gives any indication either.
The derailleur in question has RD-M752 stamped on it, but has nothing indicating the cage length. The cage itself measures 100mm c-c. Is there any way to tell?

I realize that the derailleur will work regardless, but, just for reference, would still like to know what I have.

[JiveTurkey]

100mm is the length of my long-cage XT RD.

Depending on ranges of your cassette and chainrings, you may need a long-cage.

[kenhill3]

Can't find info?

Some info from SHELDON BROWN and friends:

http://sheldonbrown.com/harris/derailers-rear.html