In Chapter 28 of Harper Lee's To Kill a Mockingbird, when Jem and Scout are attacked in the dark on Halloween night, Scout can see nothing. But she can hear sounds of scuffling, which tells us that Jem and the attacker are engaged in a physical struggle. At one point, she hears a "dull crunching sound" and Jem scream (p. 266). She runs towards Jem's scream and hits a "flabby male stomach." The fact that the attacker is obese serves as a good clue that their attacker is Bob Ewell. The attacker also tries to suffocate Scout, but he is next "jerked backwards and flung on the ground" (p. 266). At another point, the attacker starts staggering, breathing heavily, and coughing violently until he collapses--all clues signaling the attacker has just been stabbed somewhere along the abdomen or chest.
Scout next finds the road and, under the street light, sees a man carrying Jem; Jem's "arm was dangling crazily in front of him," signaling Jem's arm was broken during the fight, which is why Jem had screamed, something Scout realizes as she later tells her story to Sheriff Heck Tate (p 267).
Later, once everyone is inside Scout and Jem's home, Sheriff Tate announces that Bob Ewell was found lying dead on the ground under the tree with "a kitchen knife stuck up under his ribs," which confirms all that Lee's clues pointed to during the struggle in the dark (p. 270). During Chapter 30, Atticus and Tate have a debate as to how Ewell was killed. Atticus wants to claim Jem grabbed the kitchen knife Ewell was carrying and stabbed Ewell, but according to Scout's story, Jem was already knocked unconscious at that point; Tate realizes this very clearly. Though Tate never says Boo Radley's name, real name Arthur Radley, Tate implies it was really Boo who killed Ewell, which is not something Boo should be tried for because, as Tate phrases it, "I never heard tell that it's against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what he did" (p. 280). But Tate does not want anyone knowing what Boo did because he wants to protect Boo's privacy by protecting him from scandal. Hence, Tate decides to proclaim the story that Ewell fell on his knife.