Should a mechanic be able to tell that a cylinder head gasket is about to blow especially when they have been told that the engine is running hot?

No. He can tell you if it is blown but not if it is about to blow. Clay is 100% right. Remember that overheating can be caused by other problems or not performing routine maintainance-- which in turn may cause the head gasket to fail AND all steps should be taken to inspect/repair the coolant system. The head gasket failing or head warpage may be the result of the cooling system not working properly.