how do i tell when the ac system is fully charged on a 2001 deville? I am an old ac mechanic but have no experance with134a.

 

The only way to know for sure is to evac the system and measure the amount that came out.

Gauge readings can be tricky but generally near ambient temp for static pressure.
Compressor engaged- 40-45 psi low side 200 - 250 high side.
This should be checked at 1500 rpm and the engine at full operating temp.
If the readings are near that and duct temp is near 45 deg F then you are close to a proper charge.

All late model systems call for charging by weight and not pressure readings.
10% under or over can cause problems.