Look at the color of the oil, if it is milky colored, then you most likely have a defective head gasket. Also, look at the color of the coolant, if it has oil in it, then that is also a tell tale sign, Also, if the car keeps using coolant and there is no leak, that is a sign and if the car over heats going up hill, even on a cool day, that is a sign, or if there is sweet smelling steamy like exhaust that is also a sign. These are a few of the basic inspections with getting too deep in to this.
You can use a CO2 tester, which checks CO2 in the coolant. Sometimes you can see the bad part on the gasket, sometimes not. Usually the symptoms are enough to diagnose the cyl. head problem and do the repair accordingly.