When you are thinking about seeing a certain movie but don't know if it is any good, you will want to look online for movie reviews from those who have already seen it. Going on sites like rottentomatoes.com will give you an idea as to what the critics think about the movie you are thinking about seeing. When you look on sites like these, usually there will be an audience rating and a critic rating which is presented in the form of a percentage. If the movie that you are considering has a low overall percentage with either viewers or critics, that's not necessarily a reason to discount seeing it.
In order to find out whether or not a certain movie is worth your time, looking into a few movie reviews only from individual viewers and professional critics is probably a good idea, just so you will be able to get a sense of what they think about it. People whose job it is to review movies can identify all of the hallmarks of a bad film, which is why it's something a good idea to look at their opinion. To get an idea as to whether or not these reviews will be helpful, try looking at ones for movies you have already seen so you can a feel for just how spot on they actually are. If you find that a lot of the movies you like have bad reviews on a particular website, you might not want to use it.