To add to what other people said: As a casual user who didn't go deliberately looking for bots, I mostly caught them when they posted a comment that was a complete non sequitur to the comment they replied to, like they were posted in the wrong thread. Which, well, is because they were--they were copied from elsewhere in the comment section and randomly posted as a reply to a more prominent thread. Ctrl+F came in very handy there. (They do sometimes reword things, but generally only a couple of words, so searching for bits and pieces of their comment still usually turns up results.)
Also, the bot comments I caught were usually just a line or two, not entire paragraphs, even if they were copied from a longer comment.