The best way that you can do that is to get the product yourself and see exactly what it offers you and what it can actually do for you. When you do that you will have a better understanding of the subject( product) and you will be able to write some informative review about it.At the end of it, you will actually know whether or not this product is going to be a hit or a failure But If you don't want to spend your money in buying the product, you can just go to the vendor site, see what it offers and just write the review base in the info that you read from the site
I always write based upon my experiences with a product. It works great for me and conversions are above average.
I would research about the product (never purchase), see how others write, read articles about how to write good review, try to understand and get into customers head and then I try to do my best.
buy it or check the affiliates page, some vendors usually have written reviews you can adapt for yourself. Also you can go to ezine articles or Google and read 4 or 5 reviews of the product to give you an idea of what the product is about.