Read the essay once straight through without stopping to analyze. This gives an overall sense of the essay.
Read the essay critically this time, annotating it by making notes in the margins. Ask questions such as: What is the purpose of this essay? What is the topic. What is the main idea? How does the writer support the ideas? What kinds of words and images are used?
Make notes on the responses to the questions you asked in Step 2 and list examples from the essay. Pay special attention to the details, including vocabulary choices and sentence styles.
Analyze your notes. Review the subject, purpose and style of the essay, including the word choice and the kinds of examples used. Ask the questions: What is the attitude of this writer to the subject? How does the writer feel about the topic?
Consult a list of words that describe various tones and decide which one best fits the essay. Capital Community College Foundation Guide to Grammar and Writing lists some examples: informal or formal; light or serious; subjective or objective, to name a few. (See link in References.)