Freon leaks are found most of the time with electronic leak detectors then pin pointed with a bubble solution. The electronic detector senses the concentration of the gas and leads the tech to the highest concentration of the gas. Then the bubble solution is applied to the copper of the area of the highest concentration of gas. The solution will then bubble where the gas is leaking from the copper. Dye is sometimes used in finding freon leaks. The dye is placed in the system then it comes out anyplace there is a leak. An ultraviolent light is then used to highlight the dye to show the leaks.