Is the AC/Heater Running?

If you want a visual clue that your cooling/heating system is blowing air, just add a fan to a vent.

Print the 3d files from:

http://www.thingiverse.com/thing:479302

Insert a pin into the base, then screw the base onto a vent using the vent's side screw. Rest the printed fan blade on the pin head and the thing will spin like crazy when air is moving through the duct.