Before starting a systematic review, you may like to confirm that no systematic review has already been published or registered on your topic.
You can generally find published systematic reviews by searching common bibliographic databases. However, there are some resources with a focus on systematic reviews.
Looking at a systematic review that was published on a similar topic can be a good starting point to identify studies in your area.
Just because a review has been labelled as a systematic review does not necessarily mean that it has been carried out with the appropriate rigour. Always check the methodology to ensure it really is a systematic review. You may like to check out the Critical Appraisal tab.
Example of a systematic review with a flawed methodology: