When hot, the vapors would produce no visible leaks. Your best bet would be to pressure test your cooling system while cold and hot to ensure there aren't any leaks. Something like this: Amazon.com: Stant Inc. 12270 Cooling System and Pressure Cap Tester Set: Automotive
maybe autozone has something cheaper or something to rent.
Also, once the engine is cold you can feel around and see if there are any leaks, but as I said before the leak may only be occurring while the system is hot. You could even throw on some gloves and feel around while it is hot. Personally I think the pressure test is your best bet. Either way, start with your overflow reservoir cap, check for visible leaks and move onto pressure testing from there.