There are two methods of identifying Systematic Reviews in PubMed.
Option 1: After completing a search, you can limit your results to a predefined search strategy for Systematic Reviews. This subset is available under the "Show Additional Filters" side bard on the left side of the search results screen. Click "More" under the "Article Types" menu. Scroll down the list of options and check "Systematic Reviews."
Option 2: From the PubMed main page, select "Clinical Queries" under the "PubMed Tools" section.