When does Atticus now he lost the case in To Kill a Mokingbird?

This is all from memory and may be unreliable as a result, but I believe Atticus goes into the case with the expectation that he will lose, as he is aware of the weight of the prejudice acting against Tom Robinson.

During the actual court case itself it would appear that Tom is innocent and Atticus is likely to win but this is only based on modern perspective of the events in the book. The jury's verdict is surprising in light of this.