How to know the airspeed

1）An airspeed sensor is required. 

It is recommended plugging the airspeed sensor to the II2C port on the flight controller. You can then configure the sensor model in Mission Planner.

Here are the specific steps:
Mission Planner >> CONFIG >> Full Parameter List >> Search for "ARSPD_TYPE"

You can set the ARSPD_TYPE parameter to match your specific airspeed sensor.
Regarding the correct value to set, please consult the seller of your sensor or searching for relevant information on the ArduPilot website before making any changes.

2）If no airspeed sensor is installed, it is recommended to rely on GPS data for judgment.