How do you measure credibility of information?

True or not matters less than the emotions evoked When it comes to harm done.

For instance if a news network thinks school shooting are newsworthy what does it matter if 20 people were killed or 15? If it happened in South Dakota or West Virgina? If the shooter used an AR-15 or a AK-47 or 9mm pistol?

The only thing I think I can do is to initially make a guess, and then revise the guess according to what is being figured out later. Some things it is possible to verify by yourself, others is more difficult to do.

I double check the same news on different websites, by using Google News. I do this when the title or the content of the news expresses a sort of sensationalism.

It has never been easier to access primary/original sources anytime in history, yet people generally don't bother.