In order to identify a round character in a text, one must first understand what a round character is. A round character is one which is fully defined by the author (fully defined means the character is described physically and mentally). This characterization can be through a direct characterization (meaning readers are told directly about a character) or through indirect characterization (where the reader must infer what the author is trying to say about the character). Regardless of the use of direct or indirect characterization, a round character is one with whom the reader can relate to, sympathize with, and seem as real as a person who they know in "real life."
In regards to Harper Lee's novel To Kill a Mockingbird, many characters can be considered as round.
Scout is the most prominent character for most readers. She is fully defined both physically and mentally. The way that Scout reacts to certain things can almost be known prior to her actions in the book given readers relate to/know her so well.
Jem is another round character in the novel. Although not as much is divulged about Jem, the characterization provided is enough to allow readers to create a picture of him in their heads.
The last character one could consider a round character is Atticus. As the novel moves forward, readers and his children alike, learn about the man Atticus is. There is no question in the mind of the reader what Atticus stands for, or the extent he will go to in order to do what is right in life.