As stated earlier, the best way is to pressurize the system. Auto Zone and other such places will rent a pressure tester for $75 + tax. You get your money back when you return it. Pressure test the system when it has cooled down. After you pull the expansion tank cap or radiator cap(whatever your case may be), you hook up the tester and pump it up to about 15-17 lbs. If it's leaking it will start dropping pressure and you will see the coolant dripping down below. Now you know where it's coming from. After you think you have it fixed, pump it up again and see if it holds pressure and doesn't leak for about a 15 to 20 min. spanse of time.