The reader knows that Bob Ewell has threatened the children, and Scout smells whiskey.
Before Scout and Jem are even attacked, we have reason to believe it might be Ewell that did it. He threatened Atticus. He was said to have had a “permanent running grudge against everybody connected with that case” and that even included spitting in Atticus’s face.
Scout and Jem are attacked when they are headed home after the Halloween pageant. They have to cross a field in the dark, and Scout cannot see much because she is wearing a ham costume. She also does not have her shoes.
The children are spooked when they head across the yard alone in the dark. They think someone is following them. At first they think it is “an old dog” and then they think it is Cecil Jacobs (Ch. 28). However, there are hints that the person chasing them is Bob Ewell. After all, we know that he is after them, and the person chasing them seems drunk from the description. We soon learn there are two people out there—Boo Radley and Bob Ewell.
Our company shuffled and dragged his feet, as if wearing heavy shoes. Whoever it was wore thick cotton pants; what I thought were trees rustling was the soft swish of cotton on cotton, wheek, wheek, with every step. (Ch. 28)
At one point, Scout feels a man’s unshaven face, and smells “stale whiskey” (Ch. 28). Boo Radley intervenes, and protects her. Bob Ewell tries to attack the children, and Boo Radley intervenes, protecting them and taking Jem and Scout home.
The incident demonstrates the strength of Boo Radley, both physically and in terms of compassion. His friendship for Scout and Jem was strong enough that he came out of his house to rescue them, risking his life and taking Bob Ewell’s. All that mattered to him in that moment was that he protected them. So in the end, Scout finally got to meet Boo Radley, and bring to life her fantasy of taking him by the hand.