Ah, we just had a rousing discussion on this topic at Zoetrope. My thoughts: 1. Understand who your protagonist is, what her/his external and internal goals are, and what your theme is. You can't know if a scene serves the story until you know what your story's about. 2. Outline first so every scene serves a purpose and you aren't writing superfluous scenes in the first place. 3. Make sure every scene moves the story* forward. 4. Still can't work out #3? - Work on your objective thinking skills. (Reviewing other people's work is one way to accomplish this.) - Put your first draft in a drawer for a while. Time lends objectivity. - Get critical feedback from insightful readers. *Note that's "story" in the broad sense, not "plot" in the narrow sense. That famously contentious scene in Fargo where Marge talks to her ex-boyfriend?--Yes, you can make the case that the scene is about character and theme, which are elements of story. If you aren't a Cohen brother, it may be safer to make sure you scene addresses character, theme AND moves the plot forward at the same time. Your mileage may vary.

The questions I try to ask of any scene: Does it advance the plot? Does it reveal character? Does it provoke an emotional response from the reader/audience? Every scene ought to be doing at least two of these things simultaneously, and - ideally - hitting all three at once. If a scene ain't hitting two out of three, what the hell is it doing in your script?