Some offer discounts and gifts and such for a review. They don't even say you have to give them a good review, and so they'll justify it as an anti-bias thing. But you have to show them the review, and it's a little odd to give someone a bad review right after they offer a free gift.

The big hint is that they'll have an overwhelming number of 5 star reviews compared to the competition and often average around 4.95. Some are actually quite mediocre. So I'll usually avoid anything above a 4.8.