One option is to measure the distance between the pulley axles. I believe the gs (medium) is 80 mm? (might be remembering wrong)
Another thing I've observed is that if the triangle cutout at the very end of the derailleur goes all the way through then its the long cage. The short cage has the cutout on the outer piece only. The shimano parts diagram for m786 derailleurs confirms this. I'm not sure if it applies to older versions though.
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