A credible source is one that's written by an expert, contains accurate information that's been vetted by others, and promotes discussion of practice and scholarship.  To determine credibility, you want to consider the following:

Authority: Is the author an expert in her/his field (i.e. academic or practitioner) and affiliated with a reputable institution?

Accuracy: Are there references throughout the text?  Is there a bibliography/works cited/reference section at the end?  Scholarly articles include this section, whereas popular sources usually don't.

Purpose: Is the purpose of the resource to make money (i.e. magazines like Today's Parent) or to promote discussion of practice and scholarship? Are there colourful, glossy photos and advertisements?  If so, you've probably got a non-scholarly pulication.  Scholarly articles will often have charts, not photos.

Other:

• Is there an abstract (paragraph at the beginning that gives an overview of the article)? Scholarly articles will usually have this.
• Is there specialized terminology used in the article? Scholarly articles assume that the readers have a degree of expertise in the area, unlike magazine articles that are aimed at the general public.
• Still can't tell? Ask a librarian or Google the title of the journal and look at the "about" section of its website.  Peer-reviewed journals like to advertise that they're peer-reviewed.