Temperature IC wont actually tell you if the fan is working. Fans dont cool things, they just stir the air, they usually cool people becuase they aid evaporation. If you want to know the fan is on you either need to tell if its spinning, or if theres a strong air flow. Magnetic sensors will tell if its spinning, and how quickly. A hot wire anemometer is a good and cheap way of telling if air is flowing, a constant current source pushes a large current through a fine wire, enough to make it very hot and a voltage reading is taken over it. Put it in an air flow and the moving air cools the wire, reducing its resistance and decreasing the voltage over it. Only real problem is if the device has to last a long time, what to make the filement out of.