With a refrigeration gauge you pressure can be checked to see if there is a proper charge in the unit.

The high side gauge can vary depending on the outside temperature. The range will usually be between 175 and 250 PSI.

The low side gauge will usually be between 64 and 74 PSI.

Really good indications that the unit is running well are, the suction line is sweating and the temperature of the suction line is around 50 to 55 degrees Fahrenheit. Also the air blowing out of the condenser is warmer than the outside air temperature.