napa has a chem test kit. its a tube with a bulb & you draw the gases from the open rad cap through a fluid that will change color in the presence of hydrocarbons. the only way for hydrocarbons to get into the cooling system is a blown head gasket, cracked head or block. you can do this with a gas analyzer as well but good luck finding someone willing to risk sucking fluid into their gas analyzer.
other way to do it, use a cooling system pressure tester, pressurise the system pull the spark plugs & let it sit for a while. if fluid accumulates in any cylinder, you have found the problem. Since the combustion chamber is a much higher pressure than the cooling system can take this method sometimes won't show a problem even though it's there.
yet another way, use a cylinder leak down tester or air hold fitting. pressurise each cyl & look for bubbles in the cooling system.
when you pull the plugs look for one that is way cleaner than the others, another indicator but again not conclusive.