The freezer fan should be running at all times when the refrigerator itself is running.
Usually the freezer fan motor is getting power, but not running, due to the motor bearing going bad. You can use a multimeter SET on DC Volts and see if your getting power to it, your meter should read about 12 Volts DC.
See if you get 12 volts DC at that evaporator fan motor electrical connector when it should be running.
When you click the part link you will see a HOW TO video that shows you how to replace it.
If no 12 Volts DC, then your control board is the problem. Check your model number tag for a revision number so we can match up the control board for your model, as they make a few depending on the revision number.