Bhakti Hansoti, Mark I. Langdorf, MD, and Linda S. Murphy have published "Discriminating between Legitimate and Predatory Open Access Journals: Report from the International Federation for Emergency Medicine Research Committee" in the Western Journal of Emergency Medicine: Integrating Emergency Care with Population Health.
Here's an excerpt:
OA journals are proliferating rapidly. About half in EM are legitimate. The rest take substantial money from unsuspecting, usually junior, researchers and provide no value for true dissemination of findings. Researchers should be educated and aware of scam journals.