How do you know when to change the cylinder head gasket?

Need to use a compression tester - screwed into the holes of the REMOVED spark plugs. Then get a Haynes manual for the pressure values for your car.

a good indication that you need to do that is if you have white smoke coming from the exhaust, or if your oil looks milky.

white smoke is caused by antifreeze leaking through the head gasket into the cylinder. when it burns it makes white smoke. if your oil looks milky it is because it has coolant in it.