Fakespot used to reveal more about how they detected fakes, but as you say there are obvious issues with that, as it's a bit of an arms race. They don't just look at the text of the individual review though. Folks who buy reviews tend to get them from "review farms" that do reviews for a lot of products, and they don't have an infinite number of Amazon accounts to use for that, so there are network effects that can be powerful indicators, and that aren't easy for manipulate.