[djuosnteisn]

Flow sensor is a nice method IMO. You want to keep an eye out for two different failure modes though:
1) underflow. This is the obvious failure, either from an empty tank or clogged line or nozzle.
2) overflow. This one is a bit more difficult to detect, but not much. It protects against failure from a line / hose popping off the injector or pump and simply dumping water into the engine bay instead of the motor.

If set up and monitored properly, a flow sensor is the best option.