Visit websites designed to grade movies based on the movie rating system. These companies specialize in putting meaning to the ratings.
Read reviews and synopses of movies and compare that to the rating assigned. Decide how close that rating is to what you would have given it yourself.
Research the background of the movie rating system. During different time periods the movie rating system represented different things.
View movies that are in several different categories without your children. Although not always consistent, after several viewings you'll begin to see why certain things fall into certain categories.
Realize that movies from the video store do not all come under the current movie rating system. Movies made twenty or even ten years ago will not adhere to the present rating.
Pay attention to the fine print as to why a movie received a certain rating. Reasons are always listed next to the rating, and this gives important decision making information.
Understand that your own personal standards for you family are the best movie rating system. Movie makers are not always the best judge of what is appropriate and should not be relied upon as your complete source of information.