In case you have difficulty finding a credible source, here are some suggestions for finding information by type.
A Scholarly Article or Chapter
See Distinguishing Scholarly Articles tab for more information.
A Popular News Article
You can use Academic Search Ultimate, just change the limiter from Peer Reviewed to Newspapers and/or Magazines.
Or you could search a full-text newspaper database.
An Opinion Article Published Online
To ensure it is an opinion article, and not a news article, look for the word opinion or editorial in the section of the news source the article is from. The word may be in the title of the article, or the URL, as well.
An Article or Blog Post from a Professional or Non-profit Organization
Unfamiliar with the professional or non-profit organization responsible for the article or blog post? Google them! If there isn't a Wikipedia entry for it in the first page of results, it is clearly not an important one. Wikipedia and other websites you find will help you determine the organization's purpose and agenda.