Scientific articles found in scholarly journals are generally peer-reviewed, or referred, meaning an editorial committee has evaluated the research in the paper and the paper itself. These articles are typically long, include a list of references and report new discoveries or research results.
Many databases allow you to limit your search to only peer-reviewed articles; many but not all.
Newspapers and magazines do not contain peer-reviewed articles.
Ulrich's Periodical Directory can be used to check if a journal is peer-reviewed. Type the journal name into the search box. Click on the title in the list of results. If the category Refereed - Yes appears, it is a peer-reviewed journal.
Letters to the editor, comments and editorials are not peer-reviewed.
Some databases will allow you to filter your search results to peer-reviewed journals only. It's important to remember that database search filters are not perfect; you should still verify that the article in question is peer-reviewed.