Predatory 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 sole aim of a predatory journal is to make money, not to evaluate and disseminate high-quality research which advances scholarship in a discipline.
Charging 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 as long as rigorous review, editing, and other related services are provided.
Hijacked journals are fake websites of authentic ones, utilizing the title and ISSNs of reputable journals (Jalalian & Mahboobi).
Authors may fall prey to a hijacked journal more easily than other simple predatory journals because of the appearance of legitimacy and credentials. By falsely assuming the reputable journal's identity, the fake website is able to solicit manuscripts and pocket the money from article processing charges (APCs).
Adapted from: Jalalian M, Mahboobi H. (2014). Hijacked journals and predatory publishers: is there a need to re-think how to assess the quality of academic research? Walailak Journal of Science & Technology 11(5): 389-394. https://wjst.wu.ac.th/index.php/wjst/article/view/1004/385
If a site is not listed, it still might be legitimate; it may simply not have been evaluated. Remember that mistakes can be made, and a journal's quality may change over time. It is always best to assess a publication yourself.