Some will be obvious, such as a review for a book or game or other media item that hasn't been publicly released. I would expect a platform such as Amazon would have responsibility to suppress reviews for items that are not, and have never been for sale. A flood of reviews all coming in immediately after the product goes on sale, or a statistically improbable distribution of geographic locations would also be suspicious.
Amazon has a program (Amazon Vine Voices) that enables sellers and brands to send select customers pre-release items for review before the product goes on sale. Sellers/brands can also do that outside their sales platform. Note that these are almost always "free products in exchange for review" and are supposed to be appropriately disclosed.
