You will see a H & L on the compressor. If you follow the lines to the fitting you will know. Not sure on the model you are working on so this should help you locate the proper fitting.
One of the lines will go to the condenser in front of the radiator. That is the high side. The other one will go inside the cab, that one is the low side. You should watch both sides with the gauges and make sure you don't over charge it.
Here is a PT Chart to help you out. You want about 20* to 30* above the outside air temp for the high side.
The pressure will relate to temp, by using this chart you can figure out the temp by pressure. I am guessing you want about a 35* to 40* evap temp and what ever is 20* to 30* above ambient temp for the high.
Note that if you have a TXV system and I think you may it will control the low side pressure. So you want to be careful not to over charge. Watch your head pressure and make sure it don't go to high or you could pop a hose.
http://www.aircondition.com/pressure.htmAnymore questions?
