I do Amazon Vine reviews and we learn quickly all the things we can't say. For health products you can hardly say anything due to the legalities of appearing to make health claims. People also get their reviews removed regularly for claiming something is inauthentic. I kind of get why, because a person probably doesn't have the equipment to really determine that, and Amazon has separate channels for reporting such things. Basically reviews are just for relating your experience of a product. There are ways of communicating lack of authenticity by being more humble, as in noting that it doesn't seem like leather, or when burned it melts like plastic. I've reviewed many e.g. fake memory cards, and had no problem noting that it has less capacity than claimed, and showing some test programs' results that confirm.