Also, to answer your original question. The gauge can be hooked up to either side (assuming you have the adapters), because pressures equalize as it sits.
DONT RUN IT! It's super easy to check with it sitting and off.
Depending on ambient temp, a full system will hold X amount of pressure. If it is lower than that, it has a low charge.
Use this site to determine exactly what pressure you should be at.
http://www.csgnetwork.com/r134apresstempconv.html