My 1989 300E just came back from a servicing and A/C diagnosis (another topic!)

The mechanic stated that the head gasket is leaking. He slid his finger along the block just under the exhaust manifold and showed me his oily finger.

I'm a little sceptical because the car does not burn any oil, there is no oil in the coolant, and the vehicle runs very well. Is there a more a reputable way to determine the need for a new head gasket? Compression or leak down test? Clean engine very well and monitor? If the head gasket is leaking it is very minor now. But if it is leaking, how soon should I replace the head gasket? Will there be disastrous consequences if I defer this repair?

Thanks again,
Paul