He is in the best position more than anyone, as well as following through, so as soon as the ball was played away by the goalkeeper, who was positioned well behind his goal-line (not that this means it was a goal), the AR was positioned behind the corner flag when he raised his flag to indicate what he identified. Unless there is goal-line technology to prove otherwise, I have no problem with his advice (& I do not believe it was pure guesswork). If I saw the same incident at any level of the game without technology, assuming he answered any question I would raise in the PMD appropriately, then I would support the decision made & I would expect the same from any Observer etc.