Airflow, Superheat and Subcooling
The best way to properly charge an AC system is to use Superheat or Subcooling. Use Superheat on systems with a fixed capillary (orfice) restrictor. Use Subcooling on systems with a TXV/EXV valve. However, both Superheat and Subcooling values are useful for diagnosing certain conditions. For example, on a TXV equipped system knowing the Superheat value can help you determine if the TXV valve is functioning correctly and the evaporator coil is getting the proper amount of refrigerant.
Superheat and Subcooling may be valued independently but some diagnostic suggestions can be done when both values are available. Each are just components of system servicing and are poor indicators if other factors such as airflow, coil cleanliness, etc. have not been addressed.
To accomplish the task you will need gauges appropriate for the refrigerant and a thermometer capable of taking wet and dry bulb measurements as well as the temperature of the refrigerant lines. You will also need the manufacturers Superheat or Subcooling chart specific to your unit, usually located with the unit. The chart may also be found on the manufacturers website. In the event the chart cannot be located HVAC Buddy calculates target Superheat and Subcooling values. Again, make sure the coils are clean and airflow is good.