You can often guess who the reviewers are as well. The most obvious tell is "also cite paper X" which is their own 99% of the time.
Most of the time when an author later says (formally through the review system, or informally in casual conversation) that "I know it's this reviewer who asked me to cite their own paper" they are wrong. Their main evidence is that the reviewer asked them to cite two papers that include the same author.
But of course I can't tell them otherwise because of the confidentiality, so they keep on believing that, perpetuating the myth. Perhaps someone can aggregate some statistics on this, but I genuinely believe that authors suggesting their own papers only about 20% of the time, and when they do it's because their paper was highly relevant.
The other aspect is, that there are a some black sheep who will ask you to cite a whole bunch of barely relevant papers (often >5) all from the same group. With those you very clearly see that they are abusing the system. I suspect that these instances cloud people's judgement on the first aspect, so they always interpret the question to cite as fraudulent.
As a general advice if you suspect reviewers are try to push their citation counts up, a note to the editor (or PC if a conference) will typically help, because most don't look very favourably at this.
E.g: ask the authors to cite two papers of another researcher (as was mentioned somewhere in the thread as making it easy to guess who the reviewer is), ask to compare to results of that researcher more thoroughly, use some writing style typical of that researcher country of origin, etc.
It's especially easy if you are familiar with works from the other researchers for instance because you also reviewed their work.