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.