To check if a car fuse has power running through it, turn off all components and remove the fuse panel cover. Insert one end of the test light into the negative battery terminal while touching the probe tip of the test light to each fuse. If power runs through the fuse, the test light will illuminate, indicating a good fuse. If no light appears, it suggests that there is no voltage running through the circuit or that the fuse has blown out and needs to be replaced.