B4 has a point, bleeding these systems can be a bitch. If an air lock was left in the system it would cause overheating.
To test head gasket basics tests are, water in oil or vise versa, bubbles coming out of overflow bottle. if you want to get more serious garages will do a TK test to test for combustion gases in coolant, also a compression test and cylinder leak down test, of which both will give you a definite answer.