Skilled writers can make you think their interpretations are facts. Assumptions should be reasonable. Note errors or omissions. Is the language free of emotion-rousing words or bias. You should explore enough sources to obtain a variety of viewpoints.
Primary sources are the raw material of the research process.
Secondary sources are based on primary sources. For example, if you were researching Adenauer's role in rebuilding West Germany, Adenauer's own writings would be one of the many primary sources available on this topic. Books, encyclopedia articles, and scholarly journal articles about Adenauer's role would be considered secondary sources. Consult both primary and secondary sources when you have the opportunity. Many paper and online periodical indexes also contain records of book reviews.