An attribute that Jem has clearly demonstrated in his perception of the potential danger with the men gathered in their front yard one night, as well as in his refusal to go home when Atticus is confronted at the jail by the mob, is his perspicacity. Certainly, then, Jem is perspicacious during the trial of Tom Robinson.
Here are three examples of situations in which Jem displays his keen mental perception and discernment.
1.When Sheriff Tate testifies that Mayella was "mighty banged up," but he did not call a doctor and Atticus asks him repeatedly about not doing so, Jem's hand tightens around the balcony rail and he suddenly draws in his breath. These actions indicate that Jem penetrates beneath the obvious meanings of the statements and understands why his father has asked three times if a doctor were called; Atticus suspects that Tom is not the one who has issued Mayella the beating.
2.As Atticus continues his interrogation of Sheriff Tate, he perceives that Mayella was struck on the right side of her face. After Mr. Tate's testimony, Scout narrates that all the spectators in the court are relaxed, except for Jem.
His mouth was twisted in a purposeful half-grin, and his eyes happy about, and he said something about corroborating evidence....
Clearly, Jem discerns Atticus's purpose in cutting off Mr. Tate after he said that Mayella was struck on the right side. This evidence supports Tate's earlier statements.
3.When Mayella testifies, Scout notices the backwardness of Mayella who misinterprets Atticus's motives for questioning her father as cruel and demeaning. She objects to the judge that she does not want Mr. Finch "doin' me like he done Papa...." Scout whispers to her brother, "Has she got good sense?" The perceptive Jem suspects more than inaneness from Mayella,
"Can't tell yet....She's got enough sense to get the judge sorry for her, but she might be just---"
He is still observing at this point, but later he understands Mayella's motivation, and after the trial, he cries at the injustice dealt to Tom.