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.
Well, you should get it check by an expert mechanic. Though you have known some details about the car before buying it, there are still a lot of things that a mechanic would know that a car dealer wont. To get it running better and to prolong its life, you should consider checking everything and fixing/refilling/changing the things that needs to be done. And after that, maintain it properly to stay away from expensive mechanic…
There are two (2) torque screws that hold the cylinder itself onto the car. Remove both and the cylinder should fall out. (this is the hard way) Remove one (1) Torque screw, bottom. Then turn the key to the ON position (not running) and then press the tab that holds the cylinder in and the cylinder itself should fall out. (this is the easy way)