Excessive pressure can be due to a head gasket leak, it can also be due to overheating. There are kits you can use to test for the presence of combustion products in the coolant, but they may or may not be reliable.

Does the upper radiator hose still have pressure on it in the morning when the engine is cold? Assuming the cooling system is tight and not leaking, that's usually a pretty definitive test for combustion products in the cooling system.