Answer:
There are many different ways a head gasket can "blow". i just replaced one that was haping serious problems. in most cases you will expereince some kind of over-heating and extremely poor engine function. in my case my oil level was fine, but the gasket had ruptured next to the edge of a cylinder and was leaking coolant into the cylinder. this cylinder and everntual another failed to fire correcting and eventualy the engine wouldn't even start. coolant was visible coming out of the exhaust pipe in significant amounts. sometimes the same thing will occur only instead of coolant it will be oil leakin ing. sometimes when a gasket is blown oild can visibly be seen leaking in large amounts out from where the block meets the head and down the block. run through your engine systems and make a check list. check each and every engine system and cross them off. when everything else is fine then begin checking your main engine parts. a blown head gasket can cause many different symptoms.
If your block is cracked, more then likely this leads to catostrophic engine failure. you can visibly see a break or crack in the engine block and oil and/or coolant can be seen leaking in large amounts.
Whoever wrote the first part of this didn't mention that your engine can have a crack in the cylinder wall inside the engine and coolant will leak from the water jacket into the oil and oil into the coolant. And if the crack is in the cylinder wall you won't be able to see the crack and the oil and coolant won't leak onto the ground; but when you check your oil or coolant you will be able to see the two mixing.