What are some notable visual observations one can make of a cassette that indicate wear? And to what degree that wear has progressed?
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.