You can make a rough guess as to whether its R-12 or R 134a based on a temperature/pressure chart. If your pressure matches what R-134a should be at a given ambient temperature, that's probably what you have. The problems are 1. If your system can't hold refrigerant, obviously there won't be any pressure to check. 2. If your system has been cross contaminated (that is, has both R-12 and R-134a in it) your pressures will be higher than either one alone, and 3. As Larry said, some folk put in alternate refrigerant blends that are tough to get information on. Still, most people have R-134a or R-12, so this may help. A chart you can use is available here.