It could be a phosphorescent dye and require special equipment but most dyes you get at a auto parts store will be a visible color and will stay about forever. There is another way I've use for years and that is look for the wet dirt stain. See the Freon is not the only thing that leaks, the compressor oil does too and that collects dirt right away. Look for it. And you might note that if the Freon leaked and the oil leaked you would need to replace both. that is where people either don't replace the oil or they flood the system with to much. Just a note.
both above are correct
USUALLY PHOSPHORESCENT GREEN. THAT MEANS YOU NEED A BLACK LIGHT AND YELLOW GLASSES.
