with engine cold, remove the radiator cap, be ready with a galoon of mix antifreeze and keep filling up the radiator.

if the galoon of coolant is not enough, that means you got a major problem since the coolant went somewhere. at the shop where i work at, we do pressure testing of the cooling/coolant system to pinpoint the leak.

if you think its a head gasket problem, you could perform a compression test and it would result with 2 side by side cylinders with low compression.

hopefully its not the head gasket.