Halogen sniffer or luminescent dye with a UV light. If you've already evacuated the system, and have a means to do this and evacuate it afterwards, I'll often take compressed nitrogen, put a 250 psi positive charge on each side, and see what turns up. This is done after I know for sure there's a leak.
Chat with our AI personalities
Inject a compatible R-134 dye into the system then use a black light and look for the dye. A small leak may take days for the dye shows up.