how do they get the fuel to flow out of the pipe? is there a valve? you could put a sensor on the valve to detect flow :)
also, if it's a metal pipe, you could just use thermistors and monitor the temp of the pipe in two spots. when the fuel flows by them, both sensors should show a similar change of temp... triggering an alarm while avoiding false triggers.
actually... you could stick them close together and note the sequence that the thermistors change. if therm A changes, then therm B changes. then that would trigger the alarm. if both A and B change at the same rate, then that will just be the ambient temperature going up