Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
The tone of Othello is dominated by Iago's voice. He is the only one in the play who speaks to the audience, and his bitter rants about Othello and Cassio, his casual dismissal of women as worthless prostitutes, and his gleeful self-congratulation about the nasty things he's doing are the foundation of how we view the story. Othello even starts to mirror Iago's bitter tone in his own rants about jealousy and sexual impurity.