Even free Kindle books may have DRM. I don't know if there's a reliable way to tell if a book has DRM. The publisher may state outright that it doesn't have DRM, because it is a selling point. You may also see Simultaneous Device Usage: Unlimited under the product description, but I don't know if all of those books are DRM free. (If the simultaneous device usage line has restrictions, it will have DRM to manage those restrictions.)