The purpose can be:
Most scholarly - to inform or educate about a specific topic or to describe a research study; should be objective (it may support a particular side, but with documentation and fair consideration of the other side)
Mid-level - to inform or educate about a topic, event, or issue; might be biased
Least scholarly - to entertain, to sell you something; might be biased
Some places to look to determine a source's purpose:
Books: Check the preface, introduction, or back of the book
Magazines and Newspapers: What section is it in? News, Style, Opinion, Letters to the Editor?
Journals: Check the abstract at the beginning of the article.
Websites: Check the website's "About Us" section to find out the purpose or mission of the site. The domain (e.g., .com, .edu, .org, .gov) can also help determine the purpose of the site.
For a source you're considering: Where does the purpose fall on a scale of 0 to 10?