I scratched my head a bit and came up with the simple solution shown below.  I found an inexpensive sensitive pressure switch, which when a piece of plastic tubing was attached, could detect when the fan was blowing air. 

As shown below, the open end of the tubing was positioned in front of the fan.  The small pressure produced by the air blown into the tube was enough to activate the switch.  In theory, the switch could be positioned perhaps 10 or 20 feet from the fan.