Put nitrogen through the system, get soapy water and put on the connections etc and where it bubbles is where it leaks.
You can also push the centre of the air con compressor to make sure that's not dead also, it should spin when you're spinning the centre clutch.
Thing about car garages and air con, they do a half day course on how to operate the machine, but don't fully understand the mechanics behind how it compresses the gas to a liquid etc.
Most just also like to palm it off and blame the compressor / condenser / no gas. Just be careful new compressors can cost like £200+, so make sure they don't get it wrong.