I've seen other techs use leak detectors. The one we have at the shop is really touchy and I never use it, i'd rather use dye unless its a larger more obvious leak. similar to these (ours is really old)
http://www.amazon.com/s/?ie=UTF8&key...l_5ka58bgawy_e
The main problem I could see with using compressed air would be the moisture you would let into the system. Unless you can vacuum it out for a long period of time you may create more problems.