How to find A/C leaks, signed............A Complete Idiot

when i worked a side job, went to an auto auction with the boss. guys were at the gate selling a substitute for r12. it had propane like stuff in it. they demonstrated it and it had a leak that was close to a plug wire and the plug wire arced and BOOOOMMM SHE WENT. we used to use a propane sniffer and the r12 would turn greenish but it was making phosgene gas. very deadly stuff.