[Gregte]

You could wire an LED with a 470 Ohm series resistor directly across a fuel injector. The LED would light up to its full brightness whenever the injector is NOT firing. It will be progressively dimmer when the injector is injecting larger amounts of fuel.

Better yet, connect a voltmeter directly across the injector's two wires. Turn on your key but do not start the motor. Read the voltage. This is the full source voltage when the injector is NOT firing. Now start the motor and notice that this voltage will drop and drop further when you are accelerating (using more fuel).

Anyway, when driving, coasting, etc. just look at that voltage. If it reads the same as the key-ON, engine-OFF voltage then you know that your engine is not using fuel.

EDIT: I have that a bit backwards... If the key is ON but the motor is not running, the voltage measured across the injector will be zero volts, not full source voltage. My above numbers would be correct if you wired the voltmeter the way I did to my vehicle, which was to connect the meter between circuit ground and the switched side of the injector (not the powered side of the injector). Same applies to the LED circuit.