Running rough or sporatic overheating can be obvious clues. A mysterious loss of antifreeze with no apparent "leak" should REALLY be investigated. If you pull your oil dipstick and it looks milky and/or is overfull then that is a sign that you have water in your oil (which most likely came from your cooling system via blown head gasket or cracked head). If it DOES show overfull, let it sit for a while (all the water will separate from the oil and drain to the bottom of your oil pan) and then loosen the oil drain plug until fluid starts dripping out. If the fluid is water then you can pretty much bet on a cracked head or at least blown gasket. If you have access to a pressure tester you can see if your cooling system is losing pressure (with no sign of an obvious leak) or there is also a test that involves some special chemicals that will turn different colors if there are exhaust gasses in your cooling system. I am not sure if there is a way to tell the difference between a cracked head or block and a blown head gasket except to tear it down and look, but that doesn't necessarily mean that there ISN'T a way....I just don't know of one. Hope this helps.

Joe