Before undertaking a systematic review it is necessary to check whether there are already existing or ongoing reviews, and whether a new review is justified. 
This will require a search of databases and other resources where a systematic review or a plan for one may be indexed. If none is found the first step is establishing a need for a systematic review.
The reasons for a systematic review may include:
1. Centre for Reviews and Dissemination, Systematic Reviews: CRD's guidance for undertaking reviews in health care. 2009, Centre for Reviews and Dissemination: York. p. 292
Systematic review resources
The Cochrane Library: an online collection of databases that brings together in one place rigorous and up-to-date research on the effectiveness of healthcare treatments and interventions, as well as methodology and diagnostic tests. (Health related)
PubMed Clinical Queries: A specialised search tool that filters searches according to study categories and systematic reviews. (Health Related)
The Campbell Collaboration: Campbell systematic reviews follow structured guidelines and standards for summarizing the international research evidence on the effects of interventions in crime and justice, education, international development, and social welfare.
Joanna Briggs Institute: Supports healthcare professionals in nursing, midwifery, medicine, and allied health. (Health Related)
PROSPERO: International Prospective Register of Systematic reviews (Health Related)
Environmental Evidence Library: contains a collection of Systematic Reviews (SRs) of evidence on the effectiveness human interventions in environmental management and the environmental impacts of humans activities.
EPPI-Centre: Evidence for Policy and Practice Information and Co-ordinating Centre
Also consult subject specific or specialist databases that may be relevant to the topic area.