How to interpret high and low pressure readings when car A/C is not cooling

When a car's A/C isn't cooling, don't just look at one number on the pressure gauge. High low-pressure/low high-pressure is completely different from low low-pressure/high high-pressure. I've encountered this in my shop several times where customers say, 'Just add some refrigerant,' but once I connect the gauges, I find the high pressure won't rise, eventually tracing it to poor compressor…