What determines a classic film is the same thing that determines a classic piece of literature: the test of time. Period. No film or literature of substandard quality would ever survive that test. The key to passing this test of time is a work's universality. Since you are specifically referring to films here, let me give expand upon this idea in the realm of cinema.
My suggestion of films that have stood the test of time: The Wizard of Oz, The Sound of Music, Gone with the Wind, Snow White and the Seven Dwarves, Casablanca, The Godfather, and It's a Wonderful Life. In regards to more modern films: Star Wars, Superman, E.T., and The Exorcist.
My suggestion of films that people think will become classics, but absolutely will NOT: Titanic and Twilight. (Love the first and hate the second, but I have to admit, they have more hype than hope.)