Some times a blown head gasket will put air in the water jacket and you can hear it bubbling out after the engine is shut of and cooling. You can pressure test the cooling system to see if it leaks the other way. Pulling the plugs and checking if one looks steam cleaned is another test as is a cylinder pressure test, checking for watery foam under the oil filler cap, looking for foaming or air bubbles in rad when running and checking for coolant under the head or around the intake manifold.