DL has been very clear that if you board the flight, you lose your right to the upgrade. The best way to handle the situation is to not board if there are any seats available in F (since most elites board even if not UG'd but on the list, your position on the list goes up very quickly). I ask the GA if F has checked in full, then ask if F has also boarded full. If she says yes and I board, and see an empty F seat, I ask the FA if anyone is in the restroom. If so, fine. If not, I would go back to the GA to question him/her about the empty seat. It has to be taken care of before the boarding door closes.
But as OP stated, he wouldn't have gotten the UG anyway. In this case I say lazy GA.