My fuel level indicator is my prop... it stops when my tank is empty, and I know it's time to bring it in.

But seriously, I have thought it would be nice to get some telemetry back from the plane... RX batt voltage, fuel level, altitude, airspeed, etc. Here's a system that gives you some of these, although I don't think it does fuel level...

http://www.eagletreesystems.com/Seagull/seagull.html