Most popular search tools:
Peer-reviewed (PR) articles are also sometimes called scholarly articles, refereed articles, or research articles. PR articles are original-research articles published in PR journals, also sometimes called scholarly journals.
One complication is the distinction between the journal itself being peer-reviewed vs individual articles within it (eg. editorials, letters to the editor). If the article is a research article, describing an original study of some kind, and it's in a peer-reviewed journal, then it's safe to say the article itself is peer-reviewed.
To find peer-reviewed journals: search OneSearch and use the "Peer-reviewed" limit checkbox on the far left.
Other subject-specific databases will usually also have a peer-reviewed limiter of some kind. Medline is almost all peer-reviewed articles, so there is no limiter needed.