Besides the "audio evidence" of Boo's laughter from inside the house at the end of Chapter 4, we readers are led to believe that Scout has an innate sense of Boo Radley's presence in her vicinity. When the children are playing, she resists saying anything about Radley's existence because she doesn't want to be accused of having "Hot Steams," or believing in apparitions. Along and along, however, readers are shown that Scout has an underlying knowledge of Boo's presence, as she refuses to partake in the play that the children manufacture about the Radleys, among other things.
At the story's end, we witness the culmination of Scout's intangible relationship with Boo Radley when she is mysteriously rescued from the wicked Mr. Ewell.
The scene you are referring to takes place at the end of chapter 4. Scout is telling us about when Dill came to visit his aunt and the games the children played in the summer. One of the games was to push one another inside an old tire. When Scout nags Jem to let her have a turn, the tire accidentally rolls into the Radley yard. Scout is very dizzy and disoriented but later, when she remembers the incident, she swears she heard laughter from inside the house. Since we find out later in the story that Boo did, indeed, watch over the children as they played, it is safe to assume that Scout was right: she did hear someone laughing and that someone was Boo.