First thing I'd suggest is understand your area and what your partner should be watching. Obviously you want to focus on your area. Second thing I'd suggest is what NOT to watch. Don't watch the ball (especially on the shot). Don't watch players standing on the perimeter that are not being guarded. Anticipate situations that there is often contact--close guard on the dribbler, driving to the lane, two big guys underneath jockeying for position.
When watching contact, there are some key things you need to look for when calling a foul, and I'm sure you'll get a lot of opinions on this. Any contact that displaces, dislodges, or controls another player's motion should be considered a foul. That's pretty general.
On a fast break, you have to be aware of whether the defense is in a legal guarding position (LGP) before contact is made. Then decide if any contact (either defensive or offensive) was enough to merit an advantage.
Now I'll let everybody else point out what I missed or poke holes in how I explained it!
