First of all, I disagree with 1's assertion that this question is not philosophical, because it cannot be answered. That's typically the type of questions philosophy does address, and your question is an epistemological one, regarding knowledge.
I don't think there is a final answer that can be given for this question, but we can make progress, as others have noted above, by pointing out specific things to look for. I typically ask myself, "how could someone know such a thing?" when addressing the validity of a person's claim.
As an example, FOX News, after the latest shooting in Florida, involving high-school children, claimed that the reason the shootings occurred was because the FBI was spending too much time investigating Trump. This was a claim made repeatedly by Fox News, without any factual support. That was an example of fake news --- an outright lie to promote a political agenda. After all, how could FOX News know such a thing? They would not have had sufficient time to assess what other work the people working on the Trump investigation would have been doing. They also provided no information that would have justified arresting the shooter before the shooting occurred, based on a tip from a person who ran across a statement from the shooter on social media that he wanted to be a professional school shooter, or something like that. In the USA, it is legal to make such statements, and no one could be prosecuted for such a statement, or arrested. One could not even have their guns taken away from them for making such a statement, because that would be an intrusion on their First Amendment rights. Had the person written that he was going to gun down people tomorrow, or within the hour, then that could be a different story, but what he wrote was not at all in violation of the law in the USA. So, FOX had zero basis for its claim. There is no possible way any FOX reporter could know what they were claiming was even remotely true.