Supposing you went down to a JY looking for an M90 supercharger.

Supposing you found one, on the motor, with 150k or so miles...

How would you (or could you?) go about inspecting it, without removing it?

Also, assuming it passed inspection, what would you expect to pay for it?

Rotate the pulley by hand, make sure it turns smoothly with no grinding. If it has some slop in it, it needs a coupler. No biggie, but if you want to replace it, you need to take the nose cone off. Make sure the bypass valve moves freely, and see if the BCA and BCS are included, along with the TB. I would not pay over $100.

Only thing I'd add is part of the inspection is belt off..turning the pulley back and forth quickly to feel for the play.

What about the rotor coating...any way of getting a peek at that without removing the whole charger?

absoluetely not. the only part of the rotors you could see from the inlet would be about an inch of the ends of the rotors.