I've not seen a one that does not use links to support why they are ruling true, mostly true, false, mostly false. And they do call out false assertions from the left as well as the right. You can click a link and see a story that has links that support the the fact checkers' claims.
You can copy and paste an assertion in question and see whether there are stories that corroborate or debunk.
So the answer to your question is, YOU. You and whatever critical thinking skills, whatever education, whatever willingness to find the truth that you can bring to the table.
Who is checking the mediabias fact checkers?
Your question implies that there is no objective truth based upon facts and evidence. Which in turn allows crap like 'stop the steal' to thrive among those who simply accept a narrative of their liking. Despite guys like the moral windsock Bob Barr saying it was bullshit. Despite adjudication that said uh, uh. And despite Fox News losing a defamation lawsuit against a vote counting machine company, the fabled Cyber Ninjas coming up with more votes for Biden in AZ and the multiple assholes recently indicted for a fake electors scheme.
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