How do you know when you need more Freon in your car?

the air will be hot .

actually never, it is not an easy task, you are always taking some risk, if the system is over 7 years old it is best to rework the system. flush all old oil out , change bearing in clutch seal in front of compressor , all system seals, then refill with fresh oil and freon. as a quick remedy add freon with oil and dye and sealer mix, then pray. if you continue on though and too much oil accumulates then compressor will blow, if too little oil it will sieze and destroy system leaving a 1,200 bill to restore