There are a few different ways to tell if the air conditioning in your home is low on refrigerant. But before you jump to the conclusion that your air conditioner is low, it is important to eliminate other things that could lead you to believe that the refrigerant is low when it is not. For example, a plugged filter can cause your air conditioning coil to freeze up similar to a system low on refrigerant. It is much better to check the blower, filters and power than to assume the refrigerant is low.
Turn off power to the air conditioner. Unscrew the caps for the pressure ports attached at the outdoor unit to the large and small copper lines. The large copper line has foam insulation around it. Attach the vapor side pressure gauge hose of your manifold set to this line. Attach the liquid line pressure gauge hose to the small copper line. Usually but not always, the vapor side is blue and the liquid side is red.
Set the thermostat to cool and allow the system to run for 15 minutes. To get an accurate reading, both the indoor and the outdoor air conditioning coil must be frost and ice free. The indoor and outdoor fans must both be running and free from blockages, such as a dirty filter. Ice, airflow problems and dirty filters must all be corrected before you can get an accurate reading.
Take the temperature with a thermometer inside and outside the house. Write it down. Also take the temperature of the large copper line as close as possible to the outdoor unit by sticking the thermometer against the copper under the insulation. Clamp-on thermometers exist that take more accurate readings.
Remove the cover to the outside unit and consult the charging chart inside. There may be a table with outdoor temperatures along one side and indoor temperatures on the other side. Where your temperature readings meet will be the proper superheat reading. Calculate your actual superheat by reading the manifold gauge for the vapor side. Where the pressure is listed should also read the saturation temperature for the pressure you are reading. Make sure you are reading the saturation temperature for the refrigerant listed on the rating plate for your unit.
Calculate the superheat by subtracting the saturation temperature from the line temperature. The number you get is the actual superheat for your system. Compare this to the calculated superheat for the system. If the superheat is too high, the system may be low on refrigerant.