I have a separate cut-off device. It has a led that blinks when the voltage gets low. Once the pack hits the cut-off voltage the led goes solid and the signal to the esc is set to a neutral position so you can't drive it anymore. I call it the novak system for dumbies.
I haven't used it yet for racing, but I imagine I'm not going to be able to see the blue led very well on a sunny summer day. My local track is pretty long and I'm not sure if it would give me enough warning to get it back in the pits either. That being said, timing/ counting laps is probably the best way to know when to bring it in.