If you are interested in publishing in an OA journal and are unsure if it is a predatory journal, there are a few things you can check:
Predatory publishers engage in unfair and deceptive practices with regards to the publication of online academic journals and organisation of scientific conferences. They:
Anderson, R. (2017). Federal Trade Commission and National Institutes of Health take action against predatory publishing practices. Available from: https://scholarlykitchen.sspnet.org/2017/12/04/federal-trade-commission-national-institutes-health-take-action-predatory-publishing-practices/?informz=1 [Accessed: 2 January 2018].
There are many measures of quality to assist you in determining the legitimacy of a publisher or journal. Resources like DOAJ, SHERPA/RoMEO and recently ThinkCheckSubmit are all credible initiatives to use alongside certain indicators to evaluate publications:
These are some of the questions to ask when you decide on where to publish. For more assistance, you can ask your librarian. You can also visit specialist in copyright and scholarly communications, Denise Nicholson's tips on Scholarly Horizons.
Here is a handy graphic with tips on how to identify predatory journals:
A clip of Prof Johan Mouton speaking on predatory journals, shown at the 4th Annual UFS Research Week, 28 July 2017
How to spot a predatory publisher by the Office of Scholarly Communications at Cambridge University.