Cooling Weakly
Cool rather than cold is the usual first sign of a slow leak, and the cheapest point at which to deal with it.
Cold Then Warm
Cooling that fades after a few minutes, which is usually icing at the evaporator or a control fault rather than low gas.
Cold Moving, Warm at a Standstill
Airflow across the condenser. At speed the car does the work, stationary the fan has to, so it is the fan, a blocked cooling pack or pressures climbing.
It Was Regassed Recently
A system that needed gas last year and needs it again has a leak. It is sealed, so the gas went somewhere.
Poor Airflow From the Vents
The air is cold but there is not enough of it, which is the blower, its control module or a cabin filter that has been in far too long.
Noise When the A/C Engages
A rattle or a squeal as the compressor comes in is the clutch, the pulley bearing or the compressor itself, and it gets louder and dearer from there.
A Smell From the Vents
Microbial growth on a damp evaporator, worst on the first start of the day and worst after a long spell of short trips.
Water Inside the Car
A wet passenger footwell is usually a blocked evaporator drain, and left alone it soaks carpet, underlay and whatever electronics live under the seat.