2012 Astra Air-con

[Saleen836]

Took my 2012 Astravan in 2 days ago for an air-con recharge and the garage told me there was next to no gas in the system,thier machine ran a diagnostic and reported 'no leaks detected' so they went ahead and re-gassed the system, it blew cold but now it is blowing warm air.
I'm going to say there is a leak in the system but what is the easiest way to find it?