1995 Other Saturn Models oil leakage

The car has had a slight oil leak (more like a seep) for about a year. At first the mechanic just tightened or checked the areas where he performed the oil change, but finally he decided that it needs a new head gasket. The car has no other symptoms- no oil, coolant or water mixed in anywhere it shouldn't be, no overheating, no smoke out of tailpipe, no loss of power. The oil level is not even going down, so it is slight, but enough to notice in the driveway. He informed my husband of the diagnosis, but since my husband knows little about cars, he didn't ask any questions. The mechanic suggested it be taken to the dealer if we decided to have the head gasket replaced. But I wonder, could he be wrong about this? Is diagnosing a head gasket problem an exact science? Should I question if he did certain tests to come to this conclusion? We have no intention of paying for repairs that are more then the car is worth, but would like to know how "terminal" it might be so we know how far it can be driven. Now we are treating it like an old lady and only driving it around town. But if my son can take it college, I would like to know. I trust our mechanic to not lie to us, but I am questioning his ability to find the problem.