I’m not sure any of this is obvious. I have observed my relatives leave reviews just like the one you described as fraudulent. They are older folks who don’t get what the point of reviews is. Sometimes they will leave reviews like: „cheap and arrived in 1 day” which completely misses the point of what product reviews should be. On the other hand, I wouldn’t be so sure that „this is complete junk” review is genuine or perhaps comes from the competitor who wants to hurt the rating of the product.

Just like sarcasm is really hard to detect in an online forum, even with human intelligence, and having some conversational context, I think determining reviewers true intent is really really hard.