We normally carry out a "vacuum test", which involves putting the vehicle on the machine and sucking the system to a vacuum, this takes half an hour. if it holds the vacuum the system should be ok. However sometimes the the system holds vacuum, however the gas still leaks out.

Its not really possible to check for leaks until a charge of gas has been put through the system. We Use a gas "sniffer" which can help trace the leak, or a UV light if the dye has been put through the system.