How does Heck Tate realize that Scout was almost killed?

After Jem has been carried home safely by Boo Radley, and Scout has been removed from the ham costume she wore to the Halloween pageant, Heck Tate is able to examine the costume and sees knife marks on the outside of the ham.  It is probably the chicken wire in the costume that saves Scout’s life from Bob Ewell’s vengeful attempt to get back at Atticus.  It is then that Heck Tate realizes that Boo...

After Jem has been carried home safely by Boo Radley, and Scout has been removed from the ham costume she wore to the Halloween pageant, Heck Tate is able to examine the costume and sees knife marks on the outside of the ham.  It is probably the chicken wire in the costume that saves Scout’s life from Bob Ewell’s vengeful attempt to get back at Atticus.  It is then that Heck Tate realizes that Boo Radley was only defending Scout and Jem when he fought and killed Bob Ewell in the struggle.  Because of Boo Radley’s heroics and sad existence in the Radley house, Heck Tate decides that Bob Ewell fell on his own knife and died.  Tate believes that arresting Boo Radley would be a crime, and he explains to a shaken Atticus that it is only fitting to not arrest Boo since an innocent negro man is dead, and the man who killed him is now dead, too.  He understands the poetic justice in the moment and decides that nothing will be gained by accusing Boo of murder.