Once you've done your search, you'll see in the left sidebar "Limit To," where you'll see "Scholarly/Peer Reviewed." If you'll click that, all the remaining results should only be peer reviewed. You can watch the number of results go down to make sure.
For the most part, this filtering will be correct, but sometimes it's just the *journal* that will be peer reviewed, and a particular *article* may not be. In the individual sources, just watch for things like "Opinion" at the top of the page or not citing any sources. Those kinds of things are pretty good clues that it's not a scholarly article.
For more help check out our Narrowing or Broadening your Results guide.