How can you tell when head gasket is blow

Loss of coolant, white smoke from the tail pipe, bubbling into the reservoir when engine isn't overheating.

A "sniffer" test can be done to confirm a suspected leaking head gasket. Air is drawn from the radiator through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are leaking into the cooling system, the liquid will turn bright yellow.