What are some notable visual observations one can make of a cassette that indicate wear? And to what degree that wear has progressed? Last summer I bought a '02 or '03 Specialized Stumpjumper. 3x9 speed.  I replaced the crankset this summer with a Shimano FC-622 triple. Yes its 10 speed but I've experienced no problems and felf reassured by Sheldon Brown's experience that a "one generation difference (between crankset and chain width) doesn't affect performance." https://www.sheldonbrown.com/speeds.html
I've now replaced the cassette and chain, staying with 9 speeds (Shimano XT M770). 
The pictures above are of the removed cassette which based on the lockring is a Shimano CS-HG80-9.  The tooth counts also jibe with the numbers listed as HG80's stock numbers. The old chain is Shimano DuraAce CN-7701. Chain measures 12¹/16 per 12 links in a couple three spots. I'd like to use this cassette and chain together on another bike. What are some visual signs on the cassette that would indcicate too great of wear? What is your opinion of this cassette's wear?  Prior to my purchase of the bike, I have no history on it's use. It appears to be in good shape and the paint in vulnerable areas of underside of down tube, BB shell and fork aren't even chipped. The old crankset was likely original (strongarm II). Teeth were worn and in a couple spots either broken off at the tip or these were the normal undersized ones suffering great wear. It's life ended pretty quickly (&lt;20 miles) when the non stock right pedal reamed out the hole in the crank and fell off.  To sumerize, I've no idea on the milage or time the cassette has on it.  I'd like to use the chain/cassette combo for the rest of it's useful life.  Thoughts?
Chains wear faster than cassettes, so check the chain wear first. A simple chain wear gauge is cheap and gives a accurate indication of wear - or at least it will tell you if the chain is worn out or nearly so.

If the chain is OK then there is a higher chance that the cassette has not been used with a worn chain. Riding with a worn and elongated chain accelerates sprocket wear.

I cannot see any bad wear on the cassette. Hard to say how much life it has - it totally depends on your riding style. I say install it and try it out. If it skips or you have poor shifting try a new cassette and chain. You won't loose anything by giving this one a go.