Never release r143a into the atmosphere, that is wasteful and bad.

Use two tools, a thermometer and a pressure gauge.

Thermometer: close all of the dash vents except the center. Make sure the outside air wheel is closed on the center vent. Set the temp dials to 60F, recirc switch on. Place the thermometer over the dash vent. Start the car. The thermometer should go down to the 40Fs depending on outside temperature. Figure about a 30 degree difference.

Pressure gauge: Plug it into the port where you would add refrigerant and watch the gauge with the AC running. It should fluctuate between 20 - 40psi, but it should take longer than 10 seconds to drop to 20 at idle.

I have been known to watch the pressure lines sweat at the firewall. If one is ice cold and the other one warm, then I add some r134a until the warm one starts to get cold.

Klaus
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Common sense trumps a PhD