Fake news is not news you disagree with, in fact, it is not news at all -- it is content generated by non-news organizations to drive eyeballs to ads (e.g., clickbait) or to spread false information (rumors, conspiracy theories, junk science, and propaganda, for example).
Infographic published by The Huffington Post, by Nick Robins-Early, 11/22/16
1. Websites created to look like familiar mainstream news sites, e.g. "Boston Tribune."
2. Is it an advertisement designed to look like a news story: "native advertising"?
Look for labels: a corporate logo. Or a tiny statement indicating Paid Post, Advertisement, or Sponsored by. Or the tiny Ad Choices triangle at the upper right corner of an image.
3. Is it satirical news (for example, The Onion)?