How do I know if the head is cracked?

Best check is a test called a head check, this is a simple and quick chemical test to check for Co in the cooling system, if so this is a head problem, also bubbling in the radiator is a fair indication, you can look at the plugs and if one or more show signs of a dirty brown colour on the insulator, this is water stain, and in worst case, white smoke and steaming out the exhaust.