Probably the easiest way is to measure the temperature of the air entering the system at the filter then measuring the temperature at the closest register. A good system will show a 20-25 F degree difference in temperature. A restricted airflow will have a higher differential. This is a subjective decision. If the house does not cool properly after a technician adds Freon to the system, and the house has balanced airflow to the rooms, then I would clean the evaporator coil. Many things cause a system to stop cooling properly, but dirt is the most common problem. Many technicians will attempt to add Freon when the coils are dirty. This is because they do not want to clean the coils.