Re: Engine Damage??
Intelligent engineers can make a good assessment of wear using a spread of indicators.
1) Oil consumption (and any blue smoke from exhaust). This can indicate bore/piston ring wear and /or failed valve oil stem seals or worn valve guides .
2) You can do a compression test which checks the ability of each cyclinder to hold compression. This can give an indication of faulty valves(burnt or carboned up) or once again piston ring wear.
3) Then there is the leakdown test which is used to try and differentiate whether the loss of compression is caused by the valves or the pistons & rings.
I belive the really clever guys can also test the sump oil for products of combustion which tells you whether your piston rings are OK.
You just add into all this the subjective side, such as how the engine sounds, does it give good power, does it rattle at all on start up, and depending on how many miles its done...is this reasonable.
As to pre-empting future problems..well that starts on day one. Warm the engine up gently before using the full rev range. Change the oil (semi synth is a minimum requirement IMHO) and air filter regularly, avoid short journeys/stop start journeys and don't over rev the engine. Following that regime will minimise wear as much as is possible.
Hope this helps