Given the price of R-12, the best way to test for leaks (at least initially) would be visually -- examine the A/C lines throughout for oil patches, especially at connections. The oil is A/C compressor oil that is carried out when the refrigerant escapes. Then, I would check by evacuating (assuming that some refrigerant remained in the system) and pulling a vacuum. Pull a vacuum for about 30 minutes. Then check to see that the system will hold that vacuum for at least 2 minutes. If the visual inspection/vacuum tests work and the system still leaks, you will need a die injection along with about a pound of R-12 to find the leak which will likely be very small. Good Luck