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.

atlasproaudio wrote: 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.

I did that with a vintage but perfect looking RCA 77 and wouldn't you know it, the mic stopped working. What did I do wrong???

:lol:

Thanks but I don't think I have time to send the mic to Nashville. That breath tests sounds cool. I think that and just listening will do. After all... its only money!!!

Oh yeah. That 77 is a ribon mic. Don't breath on that too hard. Do people usually use pop filters on those and 44s? Or is that why it has such a big grille?

WCKnitting wrote:

Oh yeah. That 77 is a ribon mic. Don't breath on that too hard.

Thanks...I was kidding.