Poynter has organized an International Fact-Checking Network. People can sign up as signatories if they accept the principles espoused. It looks like — for a pilot program they have initiated — Facebook is requiring their fact-checkers to become signatories.
Here is the initial paragraph of their website:
The International Fact-Checking Network (IFCN) at Poynter is committed to promoting excellence in fact-checking. We believe nonpartisan and transparent fact-checking can be a powerful instrument of accountability journalism; conversely, unsourced or biased fact-checking can increase distrust in the media and experts while polluting public understanding.