You could get a piece of fiber-optic "cable" and monitor your taillight that way. In darkness, my own taillights (DiNotte or Nova BULL) are strong enough that a glance in my helmet mirror usually shows they're running. It never hurts to have secondaries, though. If you can clip a light to the back of your helmet, that also helps you show over the top of passenger cars in traffic, so that's one way you could go.