One Search will label articles as peer-reviewed with an icon underneath the search. If you already have an article, search the DOI in One Search and see if it is labeled as peer-reviewed. Many of the databases will also allow you to limit articles to just peer reviewed titles. If you have any questions about how to search for articles contact a librarian.
For more information about the peer-review process see The University of Berkley Museum of Paleontology's Understanding Science explanation.
(The Primo search is based on the journal listing in Ulrichsweb through an automated process, so for perfect accuracy each journal's editorial process should be examined.)