For me, it's the 4-star and 3-star reviews, and distribution of them compared to the rest (this can be gamed though). I see no reason that somebody would pay for 4 or 3 star reviews, and also competition would pay for 1 or 2 star ones instead. Primitive logic, but worked +-OK for me even on Amazon.
The chap on BBC radio yesterday said that he deliberately gave lower marks (4 star) than 5 because of exactly that - it looked too obvious.
He also said that some of the products that he was paid to review he didn't even use (hemorrhoid cream being one item).
I have also noticed the tit-for-tat reviews on Amazon between competitors. Its quite common on the cheaper products, in my experience. I've bought one or two awful things due to being caught by fake reviews, but I do tend to go back and leave a dreadful review.
