Describe the plot of the film to a friend to see if he remembers the title. Movies are a common subject of conversation, so someone in your social circle will likely be able to help you name the film in question.
Call or visit a video retailer and tell the plot to one of the workers. He should be able to name the film or he might be able to find it in the computer using plot keywords.
Visit a film website and type a brief plot synopsis into the search engine. The site will give you a list of possible matches and you should be able to locate your title among the choices. Many of these sites also have bulletin boards where you can explain a plot and see if anyone else on the site can name the film.