Jessman, take it to a mechanic and have them do a cooling system pressure check. What they will do is pump up the radiator with a gauge and leave it for a bit to see if any leakage occurs. If it's a blown head gasket, it'll leak into the cylinders and possibly shoot out the spark plug holes if they're removed. If it's anything else, you may be able to hear the leak if the shop's quiet enough.
I'd still put the 50/50 antifreeze mix in it - plain water will let it run hot quicker.
I'd still put the 50/50 antifreeze mix in it - plain water will let it run hot quicker.