In "To Kill a Mockingbird", how does Jem know that the jury has convicted Tom even before anyone can leave?
You're on the wrong track here. Jem is quite confident that the jury will not convict Tom. Right before the verdict is announced, Jem tells Reverend Sykes, "'He's not supposed to lean, Reverend, but don't fre, we've won it . . Don't see how any jury could convict on what we heard --'" (208). Clearly, Jem thinks Atticus has presented a clear cut case to dismiss the charges against Tom.
Then a few pages later when the verdict is read, Jem is shocked. Scout states, "I peeked at Jem: his shoulders jerked as if each 'guilty' was a separate stab between them" (211).
Now, you might make a case that Scout knows ahead of time that the jury will declare Tom guilty because she notices - and she says that this is something only a lawyer's child would catch - that not one single jury member looks at Tom. According to Scout, this is a clear sign that the jury has convicted him.