Some good ways to measure CFM include using a measurement from an air balancing hood and measuring the airflow with an anemometer. CFM stands for cubic feet per minute, and is used to measure how much air a ventilation system is capable of pushing.
An air balancing hood is a device that can be placed against a flat grille on a ceiling or wall. Air flows through the device as it passes through the grille, and the device measures how much air passes through it every minute. If this method is unavailable, an air measurement instrument called an anemometer can be inserted into an air duct and used to measure the average air speed. This speed can then be multiplied by the area of the duct being measured to find the CFM.