While you are proofreading the text for grammar mistakes, you could evaluate the text by asking yourself the following questions.
What is the main idea? What is the writer trying to say? What rhetorical devices does the author use? Did the writer organize his essay so that the reader will not be confused? Was the writer able to bring about his point? Did the introduction help start off the essay? Did the writer conclude appropriately? Did the writer use appropriate transitions to link ideas?
It will be hard evaluating and proofreading an essay at the same time as humans cannot multitask. I suggest doing them both separately.
