Stick your nose in the expansion tank filler, on a gasoline engine you can smell a head gasket gone. Never tried it with a diesel but i guess you should be able to smell exhaust.
Another test
From cold remove filler cap to equalise pressure, replace the cap. Start engine and give it a good rev but only for about 10 seconds. stop engine and immediatly remove cap, if you hear a gush of air escape, again it points to a head gasket as it hasn't had time to make pressure due to heat