On a lot of the Cessna 172 and 182 airplanes I have seen, there isn't an easy way to look at the engine in the engine compartment and see all of the components during a preflight. In fact, usually the only thing you can really do to see the inside of the engine compartment is to peer through the tiny oil/fuel sump door.
The POH for a 1978 Cessna 172N in the Preflight Checklist simply says:
Engine Oil Level -- CHECK
Before first flight of the day and after each refueling, pull out strainer drain knob...
Propeller and Spinner -- CHECK for nicks and security.
- Carburetor Air Filter -- CHECK for restrictions by dust or other foreign matter.
So I know that while it's not a requirement to see the components of the engine and look at the engine compartment, it seems prudent to add something to your preflight that might help give you an indication if you should investigate further before attempting the flight.
How do you more thoroughly preflight the engine of your aircraft when you can't access the engine compartment without completely removing the cowling?