Just because a review has been published as a "systematic review", it does not imply that the review was rigorous, reproducible or exhaustive. There are a number of criteria by which you can assess the quality of a systematic review. Below are the two most common tools used for this purpose.
Searching for existing systematic reviews on a topic related to your own research question can be a good place to start. These systematic reviews may provide a model for approaching your own review of the literature, including recommended search strategies and resources. The following are links to online libraries of systematic reviews and evidence documents for health and other areas of policy-making.
Sources with a focus in clinical medicine
Sources with a broader focus for public health and sociology
Sources for finding systematic reviews in other disciplines