they have a leak detector wand type thing that senses where the leak is like those radiation detectors (cant remember the name at this instant) - it will pinpoint it real quick. DO NOT use leak stop crap - that stuff cost me a ton of problems clogging the STV. I know it sucks, but I would take it to an A/C shop.. I just dropped a grand on a rebuilt compressor and having them fix all the leaks only to find my STV was failed and had to spend all weekend tearing the car apart to gut that and replace the expansion valve, then pay another 80$ to have it recharged. A/C work sucks.