For measuring velocity directly the ideal tool is a hot wire anemometer. This is a thin probe you insert in the flow. Inside the probe and exposed to the air is a length of wire heated by the instrument. Airflow cools the wire and the change in resistance is measured and converted to display velocity. Because its a thin probe it has little effect on what you are trying to measure--unlike the fan type which has a big effect on the airflow. Those cheap pocket-size fan anemometers are really only good for relative measurements in your own system. Not as a reproduceable measurement to compare with others, or with fan specs.