A garage can do a pressure test, I think they connect a pump to the header tank then jack up the pressure to locate leaks.
AWD done this to mine to locate a leak I had, ended up being a very fine hairline crack on the radiator that was even hard to locate with the pressure test but evertime I drove it, I'd get the steam from the engine bay. New radiator and all sorted. My symptoms were different to what you have though.