you just look at it. if the concave bit to the left of the top tooth is more concave than the bit to the right, then it's worn. IF it's not, then it's not.
if the teeth look symmetrical then it's fine.
Also the most used cogs will show the wear and tear more than the least used. I had a cassette on my Trek that was so worn the chain slipped constantly and was unrideable. How the previous owner let it get that bad was beyond me.
A new one tightens things up no end...especially when combined with a new chain!
The sprocket was off my work bike. There's no need to change gear on my way to work, so I didn't, and after a while the rear mech corroded and I couldn't. The sprocket was like that when I finally gave in and got a different (fixed gear) bike.
It was a 6-speed cassette hub, so the required overhaul would have needed a large proportion of new and/or hard to find parts.