Now we're beginning to understand how difficult it is.
The rule of thumb is basically "referee the defense, and if the defender didn't do anything wrong, we don't have a defensive foul." As far as trying to see both, you first hustle to get into a good angle; and you use your peripheral vision and your experience to know when the player with the ball began his "habitual shooting motion." You also focus back and forth between the two as the play allows, trying to see the whole play. That's why you see more and more often the ref making the call "half a count late." Especially on the really close plays. For in that half-second after the play, you usually can see the mental movie and thus the entire play. It's hard to describe, and certainly not easy to do. Especially in transitional play.
Another thing you look for is whether the defender is actually playing defense, or is he is just trying to draw a charge, and that's not always easy at the Div 1 level. If a guy is actually playing defense and gets run over, chances are it may be an offensive foul. Other guys are trying too hard to get knocked over, and you have to see blood before you'll give those guys a call.