How do you know head gasket is blown?

I usually hook up a pressure tester to the radiator for about 20 minutes or so...pull the spark plugs out then crank over the motor. If the head gasket is blown then antifreeze will usually have gotten into the cylinder that has the blown section of the head gasket. When you crank over the engine then antifreeze will squirt out of the cylinder with the antifreeze in it.

Also smell the exhaust.. can you smell antifreeze in the exhaust. Is there white smoke coming out of the tailpipe ? That is also a sign of a bad head gasket.

Finally. Does the car overheat after a short period of driving ? Yet another sign of a bad head gasket.