If you don't have a gauge set, just buy the freon and the cheap fill adaptor to the cans, and put one can in at a time, let it run a few min, and check the cold side going back to the compressor as it needs to be really cold to the touch, and the hot side needs to about burn your finger. depending on the humidity and ambient temp, the cold line should almost be sweating.. just my two cents...
i have the equipment... was a certified hvac person for several yrs. and for those who will complain, yes, there can be other problems in the system, and the pressures might not be exactly right, but this is the cheap way which usually works just fine.
Now... who is going to bring up the "use propane" in the system...