Investigate the expertise and agenda of the source to determine its importance and truthfulness.
Use Wikipedia. You can add the word "wikipedia" to the base of the url or the author's name in a Google search. For example, if I wanted to figure out more information about an online news source I could type "npr.org wikipedia" in the search bar to find out more information about the source outside of the source (moving beyond the "About Us" section on the website).
On social media platforms like Twitter you can use what's called the hovering technique (see the link below) to find out more about the person or organization behind the tweet.
Use this technique to learn more about people who share information online.
Move on to the next step...