View the movie first. Watch the whole movie before you decide. Think about it from your child's point of view. If it seems like it may be too mature, don't let him watch it.
Check online. Many websites offer in depth reviews of movies and give their own rating system based on gore, language and sexual situations.
Take your child's personality into account. If your child frightens easily, then a PG-13 movie with a lot of blood may be too much for him to handle. It may not be advisable to take an aggressive child to a violent movie.
Watch the movie with your child and be prepared to turn it off if it seems to bother him. Be prepared to answer questions and offer reassurances if he becomes concerned.