Air Flow Detector #1: This simple circuit uses an incandescent lamp to detect airflow. With the filament exposed to air, a constant current source is used to slightly heat the filament. As it is heated, the resistance increases. As air flows over the filament it cools down, thus lowering its resistance. A comparator is used to detect this difference and light an LED. With a few changes, the circuit can be connected to a meter or ADC to provide an estimation on the amount of airflow.

Air Flow Detector #2:

Airflow Monitor Protects Components: EDN-Design Ideas

Electronic air flow detector: