In the HVAC (Heating, ventilating, air conditioning) industry air flow 
detectors are off the shelf, but depends on the size of the fan. Might 
want to consider a solid state pressure sensor across the fan. Check out 
this link for the HVAC variety:
http://www.grainger.com/Grainger/wwg/search.shtml?searchQuery=air+flow+switch&op=search&Ntt=air+flow+switch&N=0&sst=All

>
> You can always build something that detects the wind is blowing.   
> Depending on your orientation of the fans, you can put a "flag" in the  
> airflow and optically / magnetically / capacitively / e-field  detect  
> that it is waving.   Or measure a temperature differential that  
> indicates that air is flowing.
>
> Gus
>   
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