Many databases will indicate whether articles are from academic journals, periodicals (magazines), or newspapers. Look for an icon beside the title of the article in the results list.
However, be aware that even academic journals publish articles that are not peer-reviewed. See also "What does 'peer review' mean?" for more information on how to tell if specific articles are peer-reviewed.
You can also check Ulrich's Global Serials Directory to determine if a journal is considered scholarly or not. Simply enter the title of the journal and look for information on whether the journal is "refereed".
And, as always, feel free to check with a librarian if you are in doubt about a particular article or journal!
