The first question to ask about any source is "How accurate is its information?" At the most basic level, sources should be free of spelling errors and use proper grammar. Any essay writer whose writing is slipshod may not be reliable on other matters.
Students may not be able to tell right away how accurate some claims are, but there are important indicators. References to other sources within a book or article suggest facts have been verified. The input of editors (especially in print sources) is another sign of accuracy and fact-checking.