Reviews from sites like IMDb and rotten tomatoes. As a movie or series is older, or finished, the general audience has had plenty of time to review it and if it’s fondly remembered, then it might get mentioned on here or other social platforms.
The issue with new content is that it can be amazing at first and then they release the last two episodes and ruin pretty much the entire series, eg. Game of thrones, and more recently, secret invasion.