Before starting, it’s really important that you ensure that your research question has not already been effectively answered in a recent review, and 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.