In general, for all formats:
Look at the cover, illustrations, and any special features to determine audience.
Does a review of the book reveal any potential bias?
For periodical articles:
Locate the website for the publication.
Look the publication up in the Serials Directory.
Who is the audience for the periodical? (age, level of education, specific profession, political affiliation, religion, special interests?)
Are there any illustrations or advertisements? What do they reveal about the publication?
Is the article a feature or part of a regular column? (For example, does the article appear as part of the "Comment" or "Opinion" column?)
Look at the domain: is it commercial, educational, or a non-profit organization?
Look at the graphics & illustrations (attractive to children?) and advertisements. What do they reveal about the publication?
If you can interpret HTML code: look at the source code for the site to look at author-supplied keywords (Ctrl+U)