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:
Virtue? A fig! 'Tis in ourselves that we are thus or
thus. Our bodies are our gardens, to the which our
wills are gardeners. So that if we will plant nettles
or sow lettuce, set hyssop and weed up thyme,
supply it with one gender of herbs or distract it
with many, either to have it sterile with idleness or
manured with industry, why the power and corrigible
authority of this lies in our wills. (1.3.361-368)
Othello even starts to mirror Iago's bitter tone in his own rants about jealousy and sexual impurity—that's just how pervasive (and persuasive) Iago's tone is in this play.