Follow wjm's suggestion and have a shop put dye in the system, but make sure it is UV dye. Then after a day or so, put it in a dark area and go over all connections, the front of the compressor and the condensation drain water on the floor with a black light. If, of course, you find the UV dye with the black light you've found the leak. If there is UV dye in the condenation drain water, the problem is the dreaded evaporator problem.
Don't panic yet, there are many areas where the system can leak, but if it is the evaporator, you are looking at almost 16 hours labor and several hundred dollars in parts and supplies.
Best of luck,