A peer-reviewed article has been reviewed by other experts or scholars in the field for quality and originality. To determine if an article is peer-reviewed:
Learn more about the journal your article was published in.
Last resort: In many databases you can limit your search to only peer-reviewed articles. (This is not ideal, since it will remove some relevant items, such as peer-reviewed book articles.)
Video on the Peer-Review Process (FAU Libraries):