There are several methods of leak testing an automotive A/C system...
(1) Vacuum check. When a vacuum is drawn on the system and teh vacuum pump is then turned off, it is a good practice to allow teh system sto sit for about a half hour with the gauge set connected to monitor for vacuum deacy. If vacuum is lost during this time, it indicates a leak is present but does not tell you exactly where it is located. This type of check is only useful to verify whether the system has a leak or not.
(2) To check any further for leaks, the system is going to need to be charged with refrigerant. Once it is charged and up to normal operating pressure, an electronic leak detector can be used to check for refrigerant leaks. This type of detector has a probe that is passed aroundjoint or suspected leak point; if the probe comes into contact with refrigerant it changes the sound emitted by the tool.
Here is what this type of detector looks like:
(3) The most widely used method of leak detection used today is UV dye testing. When the system is recharged, a small amount (typically about 1/4 ounce) of refrigerant oil containing a conecentrated UV dye is charged in along with the refrigerant. After allowing the vehicle to run for a while, a special high intensity UV light is then used to examint the A/C system components. Any dye that has seeped out will glow bright yellow under the UV light. This method is especially useful for detecting very small leaks that can take days or weeks for any measureable refrigerant level loss. many manufacturers (Chrysler, i know is one) add this dye right at the factory on newer models. The main disadvantage of this type of leak detection is for evaporator core leaks; since teh evaporator core cannot be seen because it is hidden up inside the heater case an electronic detector is better for detecting evaporator leaks.
Here is what a UV dye test kit looks like: