Cylinder heads are bolted on an engine to enclose the explosive forces inside the combustion chamber. They also contain the valves that let fuel inside the chamber and exhaust gases to escape once combustion is complete. Because of this, they are subjected to intense heat and are water-cooled. When they are bolted in place, a special "head gasket" is used. If the cooling system fails, the head warps and the gasket is unable to seal properly. When this happens, the engine will not run.
Clean the head by using a gasket scraper to remove the old gasket. Any residual gasket material will interfere with the measurement, so it is important to to have a completely clean surface. When working with aluminium heads, be careful not to gouge the metal.
Use a true bar, often called a flat bar, and lay it across the surface of the head. Visually inspect the head and see if there are uneven spots that show underneath the bar. Move the bar around checking in several places. Extremely warped heads will quickly show gaps under the bar.
Use a feeler gauge and try to fit a leaf of the gauge, usually .005 inches, under the true bar. Continue this process, going up or down with the gauge thickness as needed, to determine the usability of the head. If warp is detected, compare the number to the manufacturer's specifications to see if the head needs repair or can be reused without machining the surface. In most cases, minimal imperfections such as .001 inches are acceptable and the head is OK.
True bars are highly machined steel rounds or, in some cases, flat bars that are used as precision tools for measuring the flatness of objects. To get a rough idea whether a head is warped, use a flat piece of steel. Although not as accurate, it does offer a good indication if the head is serviceable.