Databases offer various search limits, or ways to limit the articles you see in your search results, so it is important to spend some time looking at the search page of a database if you are unfamiliar with it. The most common limits in a database are the full text limit (only returns full text articles in your search results), the scholarly/peer reviewed limit (only returns scholarly/peer reviewed articles in your search results), and the publication date limit (allows you to specify when you would like the search results articles to have been published).
Looking at the search limits in a particular database can also give you clues about the content of the database. Do you see a scholarly/peer reviewed limit? If not, it probably means that the database you are searching is already ONLY scholarly/peer reviewed. Nevertheless, it is good practice to review author submission guidelines to determine if manuscripts are peer reviewed. The easiest method for locating author submission guidelines is to search the Web for the journal. This is critical if your mentor is requiring use of scholarly, peer reviewed articles. You can also use Ulrichsweb to identfity if a journal is peer-reviewed. See this FAQ: Peer-Reviewed Journals to learn more.
Note: Every time you run a new (or modify) your search you will need to ensure that the limits you originally set are still in place. Otherwise it is possible that your search results might return scholalry but not peer reviewed work.