The type of domain (.edu, .org, .gov, etc.) may provide some clues about the focus of the material. However, keep in mind that scholars and other content providers may choose to maintain their materials on other domains, for example .com. Also, .edu pages can be personal webpages of faculty and students - it is not a guarantee that the source is scholarly.
Typically, a good scholarly source will provide a list of references (essentially saying what resources they used/consulted). If they are high-quality primary sources, then the website or publication is usually more authoritative.
Definition: An eye-witness account - i.e. results are reported for the first time by the authors/researchers.
Types: research articles, conference papers, lab notebooks, proceedings, technical reports, theses and studies.
Definition: A second-hand report - i.e. results are summarized, interpreted, or commented upon by others who were not witnesses or participants.
Types: review articles, encyclopedias, magazine articles and text-books.
The following sites have more information regarding the differences between scholarly and popular sources of information: