If the system contains an adequate refrigerant charge, a visual check of the system might reveal the source of the leak. Since oil is mixed with refrigerant inside the system, the presence of oil around tubing, fittings, and on coil surfaces indicates a leak. Check tightness of all mechanical fittings, since vibration can loosen fittings over time. Use your eyes and ears. Large leaks can sometimes be heard.
If the sight and sound method fails to locate the leak, use an electronic leak detector, leak detection fluid such as soap bubbles, or both, to locate the leak. The electronic leak detector method is easier and gives more accurate results than the bubble method. Normally, an electronic leak detector will be needed to find very small leaks. Make sure your leak detector can sense the refrigerant.