I am not going to get into drag racing or land speed engines. All posts from me relate to street driven mostly stock engines, ok?
You will see minimal wear on a valve seat, and the greatest wear on the valve. The seat surface is extremely hard, and is pressed into the head during the manufacturing process. Valves are a softer material and probably due to a valve being easier and cheaper to replace.
Here is a picture showing a "new" valve and below it one that has excess wear.
Cam lobe and rocker (where applicable) surfaces do show signs of wear in older engines but if the oil film has not been compromised there should not be any wear on those surfaces. Once you "wear through" the hardened cam lobe or rocker surface the engine will only last a short time. Expect to see much increase in metal shavings in your oil pan and filter.
So the rule of thumb is "most" wear comes from the valve edge getting flatter (thinner) causing the valve to sit lower and lower in the valve seat.
A lesser amount of wear comes from the seat, which will be come wider as it wears, adding to the loss of clearance.
These factors reduce the amount of free play at the thin end of the valve, and if clearances are reduced to zero, the valve can no longer close.
Here is another picture showing the valve sealing surface has reached the top of the valve. A new valve would show the sealing (shiny) area closer to the center of the valve.
Since LESS clearance reduces audible noise, you CANNOT tell if your engine needs an adjustment unless you physically check them with a feeler gauge.
If your engine gets louder, and you measure an INCREASE in valve clearance, that can indicate several things:
1) On threaded type adjusters there may have been some movement due to poorly tightened lock nuts at the last adjustment
2) The wear on the cam lobe or rocker arm was greater than the wear of the valve into the valve seat causing a NET increase
3) The valve stem got shorter...in my experience this would only occur if the valve was physically bent. I have yet to measure one that got shorter under normal operating conditions, although i have never measure a sodium filled valve. to see if they shrink in length.
Larger clearances many times indicate the beginning of a failure. If you find one valve with larger clearance, write it down. If you need to adjust valves again and the same valve clearance is again larger, typically that would indicate you have worn through your cam or rocker and i'd personally be taking the top cover off for a physical inspection SOON.
This should get the conversation started. I used general information taught by manufacturers and my own experience at the dealership. There are situations where the information i have provided would not be accurate, but again, for most street bikes, this is how it works.