The internet is a mine of information but not all information is reliable. So how do you know if a site is reputable or not?
Check the url. Government web sites often end in .gov, universities and other academic institutions end in .edu, and military sites end in .mil.
Check what the purpose of the site is. Sites that are used to sell a product will not be a reliable source of information as it is biased.
Check the author or publisher of the article. The article is also a reliable source of information if it is authored by a respected expert in his or her particular field.
Check for documentation. Reputable articles and sites never fail to cite their sources. However, you also need to check if these sources are accurate and reliable as well.