Once upon the time there was a suggestion on the Bikecurrent list to run an optical fiber from the back to the handlebars. I used that for a while. Then I ran a second pair of wires in my dynamo based system, from the rear light, back to the handlebars where I powered a small LED. Now I have a circuit that monitors how much current goes into the rear light. If all is good the indicator LED is on. If the current is too small, i.e. open circuit situation, or too high, short circuit, it goes off. I ride a lot in the darkness, so knowing that all is good and not needing to look at various side reflections and getting distracted is important for me.