2004 Silverado A/C - Bad Switch or Low Freon?

[LBJ]

For 14.5 years the A/C has worked almost perfect on the 04 Silverado Ext Cab 5.3 Vortec. It was working fine a month ago, but the other day, I turned it on and ZERO cool air. The fan blows and the compressor clicks on, but no cool air. What is the easiest way to determine the problem? Is it the low pressure sensing switch or just low on freon? I have a set of freon gauges/lines I've had since my 1977 was my main ride, but not sure where to hook them up on this "new" truck. Thanks for any tips. LBJ