It sounds to me like you blew a head gasket. That can cause bubbles in the radiator and a loss of coolant. The coolant compression test is the way to go, thats how we used to check when I worked at a toyota dealer. A shade-tree mechanic way of checking would be to remove the radiator cap when it is cold, then start and let it warm up. Once the thermostat opens and the coolant flows, look for bubbles in the coolant.