The following is a base reference for observing how a batter fouls off your fastball.
If a right-handed batter fouls off your fastball on the first base side, or back to your left, he’s usually behind on your fastball. If he fouls your fastball off to the third base side, or back to your right, he’s usually ahead of your fastball. For left-handed batters the opposite is usually the case with respect to which side he fouls your fastball off.
There are many things that can alter this simple observation and too many to itemize here. In any case, a pitcher always should start off with the easiest and most basic observations and assumptions, then tweak those observations and assumptions with complexity as such warrants.