The topic of predatory journals, including the definition and scope of the problem, can be controversial.
I prefer a definition used by Richard Poynder in discussing predatory open-access publishers as those "who clearly and deliberately trick researchers – essentially, by failing to provide the promised (or even a meaningful) service and/or deceiving them about the nature of that service, simply in order to extract money from them" (20 July 2018 blog post on Open and Shut).
The charging of an open-access author fee does NOT always make a journal predatory. Many journals may charge an author fee for open-access publication, and this practice is not automatically predatory.
Predatory journals generally exist only to collect these (often exorbitant) fees, and publish articles as an afterthought, without rigorous (or any) review by editors or peers. Their sole aim is to make money, not to evaluate and disseminate high-quality research which advances scholarship in a discipline.
Researchers sometimes receive invitations to attend or present at conferences that are not legitimate but simply covers for an organizer to profit from exorbitant registration or presenter fees.
It is important to protect your investment of time and money, as well as ensure that your presentations or attendance at conferences are actually furthering your participation in your academic field.
Whitelist: seeks to include sites which are confirmed to be trustworthy.
Blacklist: seeks to list sites known or highly suspected to be trustworthy.
"Blacklists and whitelists share the same problem in that they attempt to externalize an evaluation process that is best internal, contextual, and iterative."
- Swauger, Shea. (2017). "Open access, power, and privilege: A response to 'What I learned from predatory publishing.'" College & Research Libraries 78(11).
Understand: If a site is not included in a whitelist, it still might be legitimate; it may simply not have undergone that list's vetting process. Remember also that (a) mistakes can be made, and (b) some journals' quality may change over time. It is always best to assess a publication yourself using rigorous standards.
Other, more specific whitelists may be provided by your department, college, professional association, etc.
Understand: If a site is not included in a blacklist, it may still be illegitimate; it simply may not have undergone that list's vetting process. Remember also that (a) mistakes can be made, and (b) some journals' quality may change over time.