The author of the source is probably the biggest factor in determining the credibility of an article. For example, if you're doing research on global warming, a scientist who studies our atmosphere is going to be a more credible source than some random person spouting out their opinions. Another factor would be how they got their information. The scientist in the example above would probably be researching for a really long time before coming to a conclusion, whereas the random person is probably thinking: "It feels colder this year than it did last year, therefore global warming can't possibly be true". The only other contributing factor that I can think of would be where you find your source. It's really for someone to just throw something on the internet, while writing a book takes lots of time and effort to get all the information and get it published. Usually, books are going to be more credible sources than internet articles.
