When you are trying to identify primary sources in the sciences, think first-hand, contemporary, direct evidence or data, original research.
Primary sources in the sciences could be conference papers, dissertations, letters, lab reports, studies, peer-reviewed scholarly work all with original research. This work often includes an abstract, an introduction, methods, results, and discussion sections.
Reviews and review articles can also be helpful as these could lead you to primary sources. Image by PublicDomainPictures from Pixabay
There are several tools that can help determine if an article is peer-reviewed.