For example: Legitimate news are members of the Independent Press Standards Organisation, or have a licence to broadcast from Ofcom, or something similar. Media on social platforms will be subject to the same standards as those broadcast on television, or written in print. If videos online are 'fake news' then ofcom deal with that the same as they would on television, or ISPO the same as they would in print.
For foreign news, they could rely on similar organisations in host countries, or an affiliation with a member.
In that regulation, Facebook & Google would agree to revoke the account of the outlet in line with the regulatory organisations guidelines.
Any news outlet that is not a member of the independent organisation, or a licencee of Ofcom is considered fake news.
In my internal "how reliable is this source", that's pretty much the first check I do in my head.