My most common leak test method is UV dye. For really tough to find leaks though, the best method is to recover refrigerant, put in two oz. of R22, pressurize with nitrogen then use an electronic leak detector.

The nitrogen pressurizes the system everywhere so that leaks are more easily detected even on the low side and the R22 is MUCH more detectable by an electronic leak detector. This is a legal method because, I suppose, the EPA feels that it is better to release 2 oz. of R22 into the atmosphere than to have an ongoing leak of 12 or 134a.