How do you know which to replace condenser or compressor?

You should have the system checked by a repair shop that is certified to service A/C systems...if your ac system is not working it may be not be the compressor or the condenser at all...before replacing anything check the system for proper charge, check the metering device (either a TXV valve or an orifice tube depending on the vehicle) and check to make sure that the compressor clutch is functioning properly...and remember never disconnect any part of the A/C system without having the system evacuated first...hope this helps.