You’ll never know that an article is available for free unless you click the link at bottom of the citation that says "All # versions." Sometimes a link will appear in the column on the right—this represents a free version, but doesn’t consistently appear. Why can’t the free version of the article — from a law review website, an institutional repository, or SSRN — appear first, or appear in a column on the right that is clearly marked?
See the picture above for an illustration of where to click for the free version when you access a law review article via Google Scholar. Keep in mind that the article you want might not be available for free, but as more and more law schools establish repositories of faculty scholarship the likelihood that it is increases every day.