How is a foul ball determined?

Answer 1

A Fair ball is any ball that either 1)lands in fair territory in the outfield (including hitting the wall) 2) hits or bounces over first or third base 3) leaves the park between the foul poles or hitting the foul poles (home run) 4) is first touched by a fielder in fair territory 5)stops moving with being touched in fair territory, and 6) did not hit the batter. Any ball that is not fair, is foul.

Answer 2

A foul ball can either be determined by the foul lines, if it is hit into the foot, leg, etc of the batter, or if it hits the plate first. If a ball is hit to the side of the foul lines where the stands are located then it is foul. If it is hit where the bases are it is fair.