Remember that Scout's narrative in To Kill a Mockingbird allows for the reader to determine the actual events of the story for himself. Scout rarely (if ever) specifically calls someone a liar. During the Tom Robinson trial, most readers probably decide for themselves that Tom's testimony is the most truthful, and that Bob and Mayella are probably lying about what actually happened. Bob is particularly untrustworthy, so it is pretty easy to see that his testimony is untrue.
Aside from the trial, there are other examples of probable lies. Boo Radley's brother tells Jem that he cemented the knothole because the tree was diseased, but Atticus tells Jem that the tree looks healthy. Jem, like most readers, believes that Mr. Radley has lied to him. Dill tells a white lie when he claims that Jem has lost his pants while playing strip poker; Jem, of course, lost them on the Radley's barbed-wire fence. Dill also told many other imaginative "whoppers" over the course of the story. Even Atticus may be guilty of a lie when he tells his family in Chapter 15 that
"The Klu Klux's gone," said Atticus. "It'll never come back."
Of course, the Ku Klux Klan is still in existence today; Atticus was either quite naive about the KKK or, more likely, was only trying to calm his family's fears about rumors of a possible lynch mob.
In easier words, Mayella Ewell