Here's something I get asked from time to time: How do I know when a scene's not important? When it's just "filler?" When it's slowing down the story?
I could go all academic and tell you it's because I realize it's not advancing the plot, or revealing character.
But to be honest, I "feel" a clunker first. I don't want to read it -- I want to skip that part and "get to the good stuff." I don't think it takes a genius to realize that's not a good sign.
Of course I don't set out to write an unnecessary scene. Who wants to do unnecessary work? So I always have a purpose in mind for a scene in terms of the overall story. However, sometimes that purpose turns out to be unimportant. In my first drafts in particular, I tend to throw in a lot of things that seem interesting at the time, but later turn out to be story "dead ends." They never develop into anything vital to the story of the two main characters.
Sometimes what I want to impart in a clunker scene is simply not vital at that point -- it's too early, say, and will have much more dramatic impact later. Then I move it to a place where it will have more impact.
Or sometimes what I want to reveal is not such a Big Deal that it requires an entire scene; that information can be imparted in or combined with another scene, so that second scene is doing much more (interesting) work in terms of the overall story.
Sometimes the information I want or need to impart gets buried in a mass of inconsequential action or dialogue. I either move that information to another scene, or cut away the unnecessary clutter.
But the bottom line is, I have to want to read every single scene in the finished book.
Even if it's for the hundredth time.