Loss of power. Regular coolant loss (visible or not). Engine misfires when cold and continues to until full operating temperature is reached. Blown-out cooling components, such as the radiator and hoses, from excessive pressure.
Head gaskets are designed to seal the space between the cylinder head (top end of the engine) and cylinder block (core section of the engine), containing combustion forces while maintaining a sealed flow of coolant to the cylinder head. Head gaskets fail from: cooling system failure resulting in overheating, loose or otherwise insufficiently torqued bolts failing to adequately hold the gasket in place, old and corrosive coolant that weakens the gasket material.
Remove, inspect, and pressure check the cylinder head for warping and cracks. Resurface or replace the cylinder head. Replace the head gasket.