He has to rely on his experience, and this might help him a lot, but also deceive him in a rare particular situation. He can hardly oversee two spots on the field at the same time. In American Football it is easier. You have a huge team of referees. Unfortunately this system cannot be transferred to soccer so easily.
American football has a basicly rhythmic discourse, soccer a dynamic.
In American Football a 'down', a play, is played for some seconds until the game stops again, if a referee sees a foul he throws a yellow flag. This cannot be made in soccer. The game lives from its fluent course with dynamic outbursts that can last for minutes. If you stop after each situation for a conference, you will kill the game. In American Football those breaks are natuarally in the game.