Are you positive you have coolant not water coming out your exhaust?
If so pull and inspect your plugs for signs of coolant. Pressure test your coolant system, look for pressure drop. Inspect for obvious leaks first if pressure drop is noted. Crank over the motor with plugs out and see if coolant comes out of problem cylinder(s) if there is one.
A cylinder leak down test is another way of looking for a bad head gasket, but not too many people have a leak down tester in their garage. Procedure is similar, Pressurize the cylinder observe pressure drop, if there is check for pressure in coolant sytem or bubbles in radiator.