Many databases allow you to limit your search to peer-reviewed articles. Check the filters or limiters on your search screen for a peer-review only option. For example, ProQuest Central provides the option on its basic and advanced search screens to narrow down to peer-reviewed content only:

If you find a journal article outside of Library Search (e.g., through Google Search) and want to confirm if it is peer-reviewed, you can search the journal's website which should have webpages specifically for author information with instructions for submissions. This will tell you if the journal has a peer-review process in place. Some article PDFs may also include information about the peer review process, such as a date of submission for peer review and acceptance.