If your fans have stopped running how do you tell if a fuse or an electronic sensor is the cause?

If I knew what you were working on I might be able to help more. On most cars and trucks with electric fans the fuse can be located in one of two places. check your owners manual for the location of the fuse. If the fuse is good, your on to the next step. The temp. sensor most likely "Calls" for the fans to come on through the "Computer." If this is the case then you may have a bad Relay. The "Computer" cannot control a high current device such as a fan, directly. Most manufacturers put a Relay somewhere in the Engine Compartment that the "Computer" engages, thus causing the Fans to come on. If the Fuse is Good and the Realy tests good then you need to look at the sensor itself. In most cases trying to test the sensor causes damage. If you determine or "approximate" that the sensor is At Fault then just go to your auto parts store and buy a new one. They aren't that expensive. If you get back to me with the Year, Make, and Model, I may be able to get a little more specific.