If the system isn't entirely flat (i.e. no refrigerant pressure at all) you can use an electronic "sniffer" to at least narrow the search. Systems that have been sitting discharged for a long time, especially if they have been open to the air even momentarily, are subject to internal corrosion due to moisture in the refrigerant. Besides the condenser, already mentioned, use a sniffer to check the evaporator coil (located in the heater box). The shaft seal at the pulley end of the compressor is the common location of a hidden leak. Be sure to check all tube connections with the sniffer, too, as well as the hoses, hose crimps, and end fittings. Finding the problem is just a slow process of elimination. Since you replaced most of the system less than a year ago, the condenser, evaporator, tubes, and hoses should probably be the first places to look.
There is no need to replace the O-rings in connections that aren't leaking. That is the recommendation of both GM and the EPA. If it ain't broke, don't fix it. Of course, if you open a fitting you should always replace the old O-ring with a fresh new one of the correct size and type.
Edit: Always lubricate new O-rings with mineral oil, regardless of refrigerant type in use, before reassembly. Don't use PAG or ester oil to lubricate O-rings ... only mineral oil.
[This message has been edited by Marvin McInnis (edited 07-24-2013).]