when does scout find out boo is good

Scout first learns of the kindness and good nature of Arthur "Boo" Radley when he starts to leave gifts in the knothole of one of the Radleys' trees. Later, when Boo sews Jem's torn pants and returns them to the fence where Jem can find them, he clearly demonstrates his kindness. Later on, he protects Scout from the winter night.

In Chapter 4 Scout discovers something shiny in one of the Radley trees that have roots which extend under the street. It is tinfoil around two pieces of chewing gum. Later on, Scout and Jem find other, nicer things in the knothole. But, Scout first learns of the true goodness of Boo when he mends the pants belonging to Jem. 

One night the boys want to try to see into the Radley house, so they sneak into their backyard. Jem and Scout boost Dill up so that he can look through a broken shutter. When he cannot see anything, they try the back window where Dill sees only a dim light. Unfortunately, as Jem puts his foot on the bottom step, it squeaks, but he manages to get onto the porch, crawls to a window and looks in. It is then that Scout sees the shadow of Nathan Radley. When it crosses him, Jem puts his arms over his head and freezes. Then, he leaps from the porch and urges the others to hurry. "Fence by the schoolyard!--hurry, Scout!"(Ch.6) They barely manage to escape because Jem gets his pants caught on the wire fence and has to remove them. "He ran to the oak tree in his shorts." In the meantime, Nathan Radley fires his shotgun at the would-be thief as the children flee the Radley property.

Having heard the blast from the shotgun, the neighbors step outside. When Atticus notices that Jem has no pants, Dill lies and tells Atticus that they were playing strip poker and Dill won Jem's pants. Later, as Scout and Jim lie in the beds on the porch, she tells him that Mr. Nathan will find his pants and take them to Atticus. "When he shows 'em to Atticus it'll be pretty bad, that's all there is to it," she adds. (Ch.6) Jem tells his sister that he is going after his pants because he does not want a whipping from Atticus. When he reaches the Radley yard, Jem finds that Boo has sewn his pants, folded them, and laid them neatly over the fence. Both Jem and Scout are moved by this act of kindness.

Later in the narrative, when Miss Maudie's house catches fire, Atticus tells his children to go and stand in front of the Radley place away from the blaze. After the fire is put out and Jem and Scout return to the house, Atticus asks, "...whose blanket is that?" (Ch.8) Scout then realizes that she has a blanket on her back. When Atticus wants Jem to wrap it and return the blanket, Jem pours out all their secrets, even revealing why Nathan Radley cemented the hole in the tree. After hearing this frantic outpour, Atticus tells Jem that they will keep the blanket. "Someday, maybe, Scout can thank him for covering her up," he says. (Ch. 8)