Before starting, it’s really important that you ensure that your research question has not already been effectively answered in an existing review or clinical guideline.
You should also check that no reviews on the topic are planned or already in progress. Reviews in progress (or prospective systematic reviews) may be referred to as protocols. A protocol clearly documents what the reviewers intend to do in their systematic review.
Some sources listing current protocols:
Some sources of published systematic reviews:
It is also possible to search for systematic reviews in many key databases. For example, all Cochrane reviews are indexed in the Medline database.
Some sources of clinical practice guidelines: