When we read vertically the tendency is to take the site at face value as being reliable. The reader may use some superficial evaluation strategies to determine if the site is credible, like reading the about page, looking at the url extension (.edu, .com, .org), or by determining the site's currency. With lateral reading, the reader ventures away from the website to do some investigation.
Instead of staying with one website, Evaluate a source by reading about it on other, trustworthy sites.
Open a few new tabs in your browser to search outside of the website itself.
Start by searching the name of the website. If necessary, truncate the URL to find the sponsor/author of the website
Use fact-checking sites like Snopes or Politifact. See if there is a Wikipedia entry about the sponsor of the website.
Return to the website and scan for additional information, such as a publisher or author name.
How is your source viewed by others? Combining the information from your various searches should give you a good idea of how this website is viewed and, therefore, whether it is reliable.
The point is to look outside of the website, do not rely on how the website describes itself (such as the “about us” page).
From the Stanford History Education Group -- Civic Online Reasoning Curriculum.
John Green and Stanford History Education Group Crash Course Series.