There should be a way to see, how much fuel you got in your car. I just had a race where I was leading by 30 seconds (rainy conditions) and was sure to have filled up to 60litres (changed strategy because weather was changing), but that obviously wasn't the case. Might have added that fuel too late before the stop.
So since wheather conditions were mixed, I assumed that the "pitting" button was green, because my drivers were following the weather strategy and didn't believe they wanted to pit because fuel was low.