In To Kill a Mockingbird, when does the judge say Tom is guilty?
In a trial by jury, it is actually never the judge who gives the verdict. Instead, the jury determines the verdict via vote and passes their decision on to the judge to be announced. The same process takes place in Harper Lee's To Kill a Mockingbird.
In Chapter 21, after the endless waiting, the jury returns to the jury box and hands their votes on a piece of paper to Sheriff Heck Tate, who hands the paper to Judge Taylor. Then, Judge Taylor reads off each guilty verdict. Scout notes in her narrative that, with each guilty verdict read, Jem jerks from the shock:
Judge Taylor was polling the jury: "Guilty ... guilty ... guilty ..." I peeked at Jem: his hands were white from gripping the balcony rail and his shoulders jerked as if each "guilty" was a separate stab between them. (Ch. 21)
Later, in Chapter 23, when Atticus is discussing the case with his children, he explains that Robinson has a good chance of appealing the trial, being retried, and acquitted. It's also at this point that we learn the jury sentenced Robinson to be executed. Jem protests, saying that "the jury didn't have to give him death--if they wanted to they could've gave him twenty years" (Ch. 23).
Hence, from the passages above, we learn that it is the jury who gave the guilty verdict and sentenced Robinson to death, not Judge Taylor, which is the standard process in jury trails.
Judge Taylor reads Tom's guilty verdict towards the end of Chapter 21. Immediately before reading the verdict, Scout notices that the jury refuses to look at Tom Robinson, which foreshadows the impending guilty verdict. Sheriff Tate then hands Judge Taylor a piece of paper with the jury's final decision written on it. Judge Taylor then reads the verdict by saying, "Guilty...guilty...guilty...guilty..." (Lee, 129). Scout mentions that each "guilty" seems like a knife stabbing Jem between his shoulders. Following Tom's verdict, Jem bursts into tears and repeatedly says, "It ain't right" (Lee, 131). Jem and the other children lose their childhood innocence after witnessing racial injustice firsthand. Despite the lack of evidence and conflicting testimonies of the Ewells, the jury convicts an innocent man simply because he is African American. Although Atticus defends Tom to the best of his ability, he is unable to influence the jury's overwhelming prejudice. Tom is convicted of a crime he did not commit and is given the death penalty.