When you are trying to identify primary sources in the sciences, think first-hand, contemporary, direct evidence or data, and original research.
Primary sources in the sciences could be conference papers, dissertations, letters, lab reports, studies, peer-reviewed scholarly work all with original research. One quick way to determine whether you are looking at a primary source to to look for the IMRD format -- the paper should include the following: Introduction, Methods, Results, and Discussions.
If any of these components are missing, you are not looking at a primary source. Reviews and review articles can also be helpful for gathering background information and can be a great way to find primary sources (Hint: Check the reference section of reviews for a list of the primary sources used to write the review).
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There are several tools that can help determine if an article is peer-reviewed.