That is to say, I do not get why a significant number of people would be willing or foolish enough to submit their manuscripts to pretend-journals, by extension how these pretend-journals can be profitable enough to the spammers for them to continue spamming (many of these "journals" have few to no articles and sometimes the few articles they have are plagiarized from elsewhere so presumably they cannot earn a lot of fees), and finally why anybody would find it hard to know the good journals in their field.
When you start out as a scientist (or scholar) you generally obtain a decent university education and write your thesis under a hopefully competent supervisor. The supervisor will be able to teach you not only how to write a decent paper but also to what journal to submit it. By extension, they will teach you what the good journals in the field are and what kind of "publication strategy" is expected of you if you want to become a career scientist/scholar in the field. In addition to your supervisor, can talk with other colleagues and learn from them. Finally, the papers you needed to read to prepare for your thesis as well as the reference list of said thesis will also give you a clear idea of where the relevant literature in your field is published.
It is true that the decision of where to submit is not as easy as looking up a few stats in Google Scholar Metrics or Thompson Reuters JCR - there are many dimensions to it. Stats like the impact factor of a journal are, of course, one of them: all else being equal, it will serve a career scientist best to publish a paper in the highest-ranking journal they can possibly get it into. But all else is usually not equal. First, the right audience is also a consideration. If you want your paper actually read and cited, it might be better to submit it to a journal that is read by all the people who might be interested in your work instead of a slightly higher ranking journal that only few of them read or have a new issue alert for. In some fields there might also be a handful of journals that you simply have to publish in for your colleagues to take you seriously as a peer.
The quality, general importance and novelty of your research are other determining factors. The highest impact journals with a general scope are, sad but true, generally less interested in the quality of the study but in the novelty: is what you have done likely to be the big thing over the next five years? Consequently, if you have a high quality but less innovative paper you are better served with the best specialist journals. But then if your study is unconventional, has run into methodological difficulties, could not be quite as perfect as you would have liked it to due to budget or time constraints etc., you may have a hard time getting past peer review in those and you have to settle for less competitive journals.
Of course, there is also high quality and important work that due to its very nature simply never gets into high ranking journals, and everybody in the field knows that. Plant taxonomy - the description of species as new to science, monographs, revisions etc. - is generally conducted in journals with impact factors from "not available" to one, but that does not mean that those journals are not widely accepted as high quality publication venues in the relevant taxonomic community. Impact factors aren't everything. Finally, there are the desirability of open access versus the affordability of publication costs to be considered.
So, to summarize: I know the relevant journals in my field because they are the ones in which I find the quality papers that I build my own work on and that I consequently cite in my own work. If I haven't got any experience with a given journal, I may also ask colleagues who have what they think of it, its editors, the quality of its review process and the time to publication. I also look up all the JCR impact factors of the major journals in my field whenever they are updated (around the middle of each year). Again, they aren't everything, but they give a ballpark estimate on how difficult it might be to publish in a given journal. All that is how I can evaluate the publication lists in other people's resumes.
When I then myself have a manuscript in preparation, this is what I do: Taking into consideration the topic, quality, novelty, general interest and intended audience of the study, I narrow the number of potential journals down to a handful. If I come up with too few, especially because the study was about something that I haven't done before, I ask around among colleagues who have got more experience in that subfield and I search for likely-sounding journal titles in Thompson Reuters JCR or Google Scholar Metrics. Then I take a closer look at the scope of those journals to see if they should be considered. I then basically group the shortlist of journals by their impact factors and submit to the highest ranking one that I consider realistic, working my way down the list if the manuscript gets rejected. That being said, I also prefer to try out journals that I have never or rarely published in before instead of always submitting to the same few, so a difference in impact factor of less than 30% may easily be outweighed by my interest in diversifying the journal names in my publication list.
Of course, some might ask what they are supposed to do if they cannot get in any of the serious journals in the field. Well, there are basically three possibilities:
- Get better at study design and paper writing.
- If the person in question considers that to be too hard, they might want to reconsider whether professional science or scholarship is really the right career for them. There are many other professions that are equally satisfying.
- If the person in question is convinced that they do not get into the serious journals only because the established authorities in the field form a cabal to suppress all dissent and are afraid of the revolutionary new theory they came up with all by themselves, I fear I do not have much to recommend. At least no advice that would be listened to.