One of the indications that a paper has been published in a predatory journal is a very short review time.
What does very short mean? It could be almost by return, but anything less than a week (or even a month) should be a cause for concern. Like most indicators, a very short review time is not definitive evidence that the journal is predatory, but it should raise a red flag and be worthy of further investigation.
You might ask why a week should be a cause for concern? Surely it should be possible to review a paper in a week, and you would be right. But it is unusual and it is even more unusual for 2-3 reviewers that are reviewing your paper to all complete their reviews within a week.
This is why you often have to wait 2-3 months (often longer) for a paper to be reviewed. The complete set of reviews is only as fast as the slowest reviewer and, in our experience, that is often several months. From personal experience, and knowing the journals that we target, we do not get concerned for six months, frustrated yes, but concerned no.
However, unless you submit a paper to the journal how do you know the length of the review time? There may not actually be a way of finding out, short of starting to email authors and even then they may not tell you as they may be embarrassed to tell you. In any case, it is time consuming and for what, just to find out what you already suspect?
If you are fortunate, the journal will maintain some sort of metadata. This will tell you when the paper was submitted, accepted and published, or information similar to that. This data, if present, will either be on the paper’s web page on the paper itself.
You may wonder why a journal would show this type of metadata? Well, some authors will be interested in how quickly they can get their paper published, even if they know the journal is predatory. In this respect, the metadata can be seen as a marketing tool.
But, it’s buyer beware. If a predatory journal publishes metadata about review times, can you trust that data? After all, the same journal is likely to be less than truthful about the reviewing itself, where the journal is indexed and the truthfulness of the editorial board.
Unfortunately, unless you actually submit to the journal you can never be sure how long the review times are. You can make some educated guesses, but never be totally sure.
The only advice we can offer is to look at what the journal says. Many legitimate journals will give indicative review times of (perhaps) 90 days, which seems reasonable. If a journal is offering review times of a few days, or even a few weeks, we would walk away and look for another journal. As we have said before, the world is not short of journals so you can easily look for another outlet for your research.