The Protocols of the Elders of Zion, considered the most republished antisemitic text in the English language, was established as a forgery because it plagiarized older, fictional texts. The claim was that the document recorded a Jewish conspiracy to control the world. Excerpts first appeared in serialized form in a Russian newspaper, the Banner (Znamia/знамя) in 1903. In 1905, it was published in full as the appendix to a Russian book The Great in the Small: The Coming of the Anti-Christ and the Rule of Satan on Earth by Sergei Nilus.

The text was not simply created out of thin air to fabricate evidence of a non-existent conspiracy, however. Instead, the Protocols plagiarized sections of an 1864 French satirical novel Dialogue in Hell Between Machiavelli and Montesquieu by Maurice Joly. One chapter of the Prussian novel Biarritz by (1868) Hermann Goedsche also provided some source material. These texts did not explicitly mention Jews, but talked of conspiracies more broadly. The Protocols inserted Jews into the preexisting framework of conspiracy that these works of fiction offered.

The fact that the authors of the so-called Protocols (who remain unknown, but were almost certainly working under he supervision of the head of the imperial Russian secret police, Ratchovsky) had stolen from preexisting texts was confirmed in an expose by the Times of London in 1921. Then in 1935 a Swiss court confirmed this in a ruling. In 1964, the US Senate issued a report confirming the Protocols are a fabrication.

So tl;dr they are a known forgery because they are a very sloppy plagiarism of preexisting works of fiction.

Sources:

https://www.ushmm.org/wlc/mobile/en/article.php?ModuleId=10007058

https://www.ushmm.org/wlc/mobile/en/article.php?ModuleId=10007244