You are reading critically when you are doing an examination of the book in every possible aspect, from the purpose of the author, why did he chose those facts, etc you are basically gathering evidence to recognize that particular perspective and meaning. ;-)

It is like not taking the author's writing for granted. You are neutrally reading the material and forming your own opinion based on observation, analysis and reasoned judgment. You could both agree or disagree with the writer. It is an unbiased evaluation of the material, the subject matter of critic.

To evaluate the reading.

Yes, you analyse as you read, instead of simply accepting what you read.

Critical reading means reading in order to gain a profound understanding of the text. While doing a critical reading, you analyze and evaluate what you are reading, i.e, the words, the sentences, the plot, the history, the writing style, the authors's bias, etc. In short, reading critically is an attempt to understand not only the meaning of the story, but the reasons why the story was written the way it was written.