Several databases have a check box or filter to limit your search results to peer reviewed research.
Primary research presents original research methods or findings for the first time. A primary source is a document (or object) which was written or created during the time under study, by someone who was present during an experiment or experience, and offers an firsthand view of a particular event. Especially in the sciences, an "experience" or "event" includes a research study or process.
In science, primary literature is the original publication of a scientist's new data, results, and theories.
A good way to evaluate whether or not a scientific article is a primary source is whether the article has a Materials and Methods (or similar) section. If the article discusses methods, it is likely primary literature.
Secondary literature does not report new findings, and includes review articles and most scholarly or academic books, including textbooks.