Reverse image search
Google Images, TinEye, Bing Visual Search
Finds earlier appearances of a photo. The single highest-yield check available, because relabelled real photos are everywhere.
Who runs it: Free tools operated by the search companies themselves.
Wikipedia, used correctly
wikipedia.org
Not a citation, but excellent for the source-investigation move: who owns an outlet, who funds an organisation, when it was founded. Check the references at the bottom rather than the prose.
Who runs it: Wikimedia Foundation, a nonprofit.
Snopes
snopes.com
Long-running fact-checking outlet. Strong on viral claims, images and urban legends. Publishes its methodology and a corrections policy.
Who runs it: Advertising and reader contributions; ownership history is public.
PolitiFact
politifact.com
Political claims, with a rating scale. Reading the evidence section matters more than the rating itself.
Who runs it: Poynter Institute, a journalism nonprofit.
FactCheck.org
factcheck.org
Political and health claims, notably careful with numbers.
Who runs it: Annenberg Public Policy Center, University of Pennsylvania.
AP Fact Check and Reuters Fact Check
apnews.com, reuters.com
Wire-service verification desks. Useful because their core business is being right rather than being fast.
Who runs it: The news agencies themselves.
AllSides and Ground News
allsides.com, ground.news
Show how the same story is covered across outlets. Useful for the framing exercise. Their own bias ratings are judgements, so treat them as a starting point rather than a verdict.
Who runs it: AllSides is a company; Ground News is a company with a subscription model.
Media Bias/Fact Check
mediabiasfactcheck.com
Broad database of outlets. Widely used, and worth knowing that its ratings are produced by a small team rather than a formal panel, so it is a signal and not an authority.
Who runs it: Advertising and subscriptions; run by a small independent team.
Inclusion is not an endorsement of every verdict any of these organisations has published. It means they are identifiable, they say who pays for them, and they can be checked against each other. That is the bar.