What do i do.My car overheated and now it seems fine

I have a 98 honda civic with 120k miles on it and was driving it on the freeway when i noticed the temp gauge all the way up. I drove it an additional 5 miles to my work and turned it off. Could not see where it is leaking from. The next day i filled the radiater with water and drove it to work ( 9miles one way) and it did not overheat. I have been driving it with water in the radiater filling it before i drive for 2 days and it has been fine. I checked the radiater itself, hoses etc. And do not see any leaks. My check engine light went on at the time of the overheating and is still on. My question is how do i diagnose the problem myself without paying labor fees to a mechanic. Just need to find out where the leak is.

It could be a cracked or leaking head gasket. This will generally cause combustion gases to leak into the water jacket producing gas bubbles and occasionally air locking portions of the cooling system. This will cause the car to overheat and it could also cause water to leak into the combustion chamber and escape via the exhaust and is often an intermittent condition. This could be detected by carefully watching for steam vapor leaving the exhaust pipe, but the products of normal combustion is a very small amount of water vapor. You can sometimes smell burning antifreeze in the exhaust and this is a sure sign of a combustion leak. You may also determine if this is a problem by removing and inspecting each spark plug. Spark plugs should be ashen gray in color. A spark plug that has a strange brownish build up on it is often a sign of water getting into the cylinder.

It would be relatively inexpensive to have this diagnosed at a shop. A shop would likely run the car with a combustion gas detector in place of the radiator cap and/or do a leak test in which they pressurize the cooling system. Call around and talk to a few and see what they would charge for such a test.