Is it when the dial hits the bottom or when the yellow bar begins to run out which is to the right of the "E" symbol on the fuel gauge?
I have never seen the yellow bar run out but it would make sense.
A good indication that it has run out of fuel is when it splutters and stops. At this point you should exit the vehicle, swear profusely and loudly whilst making hand gestures towards the car. Then kick the wheel, call home and tell them you're going to be late, and get a taxi or start that long lonely walk home.