Shiloh movie

I liked when Marty´s dad came in and hit Judd, but how did his dad know to help Marty?

I think you are referring to the Shiloh movie   They changed the ending a bit to add more excitement.  I’m not sure where the Dad was.  I thought he wasn’t home but then he suddenly appears  I guess the reader is supposed to think that while Marty and Judd were arguing, the dad had come back home and heard what was happening.