Evaluation is a higher-order thinking or critical thinking skill. It requires that students first have an understanding of the book and then apply their understanding.
In the case of Animal Farm, there are two main areas of evaluation I would focus on: the author’s technique and judging the characters.
For evaluation of the author’s technique, I would first familiarize the students with the basic persuasive techniques of ethos (credibility), pathos (emotion), and logos (logic). After all, Orwell is trying to persuade us too. For example, he uses ethos and pathos with animals. We have a certain amount of respect for certain animals, and they pull at our heart-strings. I would ask how Orwell uses each of these and give specific animals as an example. Include general questions about whether or not each technique is effective.
For evaluation of the characters’ behavior, I like to have students compare and contrast the leadership styles of Napoleon and Snowball and decide who is a more effective leader, regardless of which character they agree with. You can have students use a Venn diagram. You can also choose specific events from the story, such as Napoleon negotiating with the humans, and ask them to judge the event.