Types of sources
Articles prefer, in rough order: the Oxford English Dictionary; Merriam-Webster; Cambridge Dictionary; Oxford Learner’s Dictionaries; historical dictionaries; Google Books, Internet Archive, and HathiTrust scans; newspaper archives; library and university collections; digitized primary texts; peer-reviewed linguistic or historical research; and established etymology references.
General-interest blogs, anonymous idiom sites, discussion boards, and commercial marketing pages may document a popular story or current usage, but they are never the only basis for a strong historical or medical claim.
How claims are checked
Dated texts can show that an expression was in use by a particular time. Reference works may report earlier examples whose full text is unavailable. A citation to a popular origin story does not by itself establish that story as fact.
What to do if a source is wrong
Report it through the corrections page. The article is rechecked and the correction is logged.