If the kiln is firing longer than usual, how can you tell if it is overfiring?

It is useful to observe the color of light under the lid or door and around the peephole plugs during each stage of firing. If you have a digital kiln, notice the temperature for dark red, cherry red, yellow, bright yellow, etc. Mentally note the color of the firing chamber when the controller or Kiln Sitter trips. Then if for some reason you ever have to turn off your kiln manually without the aid of cones, Kiln Sitter, pyrometer, or controller, you could do it by eye alone.