Predatory journals copy the appearance of legitimate ones closely enough that a call for papers tells you nothing. Six checks separate them, and none needs subject expertise. The first is that a genuine journal in your field rarely emails you asking for a manuscript.
Check the editorial board against reality
Pick three named editors and search for them independently. Predatory journals list real academics who never agreed to serve, and a surprising number of those academics have public statements asking to be removed.
Check the fee and when it is mentioned
Article processing charges are normal in open access. What is not normal is a fee that appears only after acceptance, a fee that is negotiable, or an invoice arriving faster than a review could plausibly have been completed.
Check the review timeline against arithmetic
A promised decision in seventy-two hours means no peer review happened. Finding two qualified reviewers, obtaining their agreement, and giving them time to read a paper takes weeks in any field. Speed here is not efficiency; it is the absence of the thing you are paying for.
Check the indexing claims one by one
Do not read the logos on the journal's own site. Go to each index and search for the journal there. Claimed indexing that does not survive that check is the most reliable single indicator, because it is a claim the journal cannot control.
Check the scope
A journal that publishes machine learning, cardiology, medieval history and civil engineering in one issue is not multidisciplinary. It has no field, and therefore no reviewers qualified to judge anything in it.
Check who else is in it
Read three published articles. If they are poorly written, unproofread, or clearly outside any reasonable reading of the stated scope, your work will be judged by the company it keeps.
Why it matters more than the fee
The money is the smaller loss. A paper published in a discredited venue is difficult to publish again elsewhere, since it is already in print, and it stays attached to your name in every database that picked it up.
Academic Publishing