I think it is important to take everything you read online with a grain of salt. If you do not know the person writing the review, you cannot be sure of their integrity.
That said, you should feel confident that most of the reviews on Amazon are written by real people with their own opinions on the product. Some could have a personal connection to the company involved or stand to profit financially from their review, however. There are a few things to look for that might indicate that a review is legitimate.
A review that includes a lot of detail and more than just one or two sentences is typically legitimate. In addition, look for reviews that go over both the pros and the cons associated with the product; this indicates that the person writing the review is being realistic. Finally, if you are concerned about a review, look to see if the person in question has penned other reviews. If they seem to share honest opinions for other products, you can feel more confident about the review you are interested in.