An author's tone is basically the "voice" or attitude toward the subject of a passage and is often directly related to the purpose.
A useful way to determinate an author's tone is to envision the sound of the author's voice, as if he/she were reading the passage out loud.
An author's tone is often indicated by adjectives, such as;
Cynical, depressed, sympathetic, cheerful, outraged, positive, angry, sarcastic, prayerful, ironic, solemn, vindictive, intense, or excited.
