Diagnosing a damaged cylinder head gasket

[Sharky]

My car has been suffering from overheating under high load. I suspect that it might be an electrical problem, particularly with the fan as I think that instead of engaging second speed it might be shutting down. However, my mechanic believes that the cause could be a "burned" cylinder head hasket. However I wouldn't like to dimount half the engine just to find that it was OK so he told me that there are a couple of ways to tell if there's any damage. First is to open the expansion bottle when the engine is cold (preferably during the mornings) and see if there's any air going in or out (like when one opens a soda bottle). Number 2 to check for traces of moisture on the oil cap. I've done both but both tests have yielded no indications of a bad gasket.

Is there anything else that I could do diagnose it?