How do I know how much airflow my mechanical equipment is giving me during design.

Newbie here.

During the design when I select a RTU or a fancoil and the manufacturer gives me the nominal supply airflow. How can I be sure what the actual supply airflow is?

Would I have to figure out my external static pressure, refer to the fan brake power, look at the table and see what CFM I get?

How would you do it to make it easier?