I would send it in to Tracy Korby at Korby Audio Labs, testing used microphones should be left to the microphone experts especially if you are going to drop over $1000 on a used microphone. He has worked on several high dollar vintage Neumanns for me and I've been extremely happy with the service.
As for a DIY guide all I can offer is this:
Test it through actual usage and see how it holds up.
Breathe out hot humid air onto the grill/capsule and see if the sound fades out at all, if so, the capsule needs to be checked out by a pro.
NOTE/EDIT: I'm talking about condenser microphones with the breath test. Ribbons generally aren't referred to as having 'capsules' (at least not in my experience)....they have ribbons not capsules. A wind gust can damage most ribbon microphones (except for the Royer 121 or 122 which have a thicker ribbon element and are more durable but still not unbreakable). But if you don't know this already you probably shouldn't be buying used high end microphones.