forester overheating

I have a 98 forester with 150k that I have been having problems with coolant loss and overheating. There is no evidence of an external leak. I see alot of postings on head gasket problems with the subaru's. Is this my likely problem or could i have serious engine damage. I had the car to the dealer and the machanic said if you overheat these cars just once the block will crack. I find this hard to believe because the car runs perfectly otherwise. Any information will be appreciated