First off, a "foul tip" actually only refers to a ball that's barely touched by a swung bat, which travels sharply and directly to the catcher's glove (or bare hand... OUCH!), where it ends up caught. Anything else is a foul ball.
If we're talking about balls actually batted into play, there are two things to consider.
#1. Did it hit the batter while he was still in the box? That will always be ruled a foul ball, even if the ball is technically in fair territory, which brings us to...
#2. A significant piece of each batter's box is in fair territory. The foul lines cut right through them, and if it's just a typical batted ball which the batter-runner avoids contact with, only those lines (not the lines of the batter's box) matter for the fair/foul call.