I use an inverted telescope to tell me when a short story or a chapter is complete. It's personal taste, I know, but since I enjoy stories with beginnings, middles, and ends, I try to write that way.
With a short story (and some of mine are quite short) that means I want to convey more than a moment of emotion. I want to know where a character started and where she ended up.
Flash fiction deserves a bit more freedom. If a piece of flash grabs me, I don't mind rereading it once or twice, to let it sink in. Much as I do with a poem, when I find one I enjoy.
But with short stories, or chapters in a novel, I need more structure and I use the inverted telescope test to tell me when I've got it. Imagine looking at a chapter (or a story) through a lens that lets you see the whole scene at once. When setting, characters, and plot fit together in a snapshot, it works. If they don't, I rewrite until they do.
If a character looks lost, like he doesn't belong in the scene or has too little to do, I try to find a way to move him out. Or when the setting lends nothing to the story (is generic, uninvolved with what's happening to my characters), I'll search for a new one.
Imagine two characters sitting in a kitchen. While they're "doing plot"—hatching a plan, complaining, arguing, confessing love—one of them may serve the other food. But the food, the way it's eaten, what is said about it, will reveal character. Or, better, it will be part of the narrative, connect in some way to what has come before or what will come later. That can be a lot to ask of a buttered bun, but imagine a scoop of refried beans smothered in jalapeno salsa. Or a long-forgotten bottle of wine drawn from a cellar.
For me, the picture is complete when all three fit together: setting, plot, and character. Once I've got it, and can tell it self-contained, that piece is done. And I can move on to the next.
With thanks to Deryn Collier for making me think about structure. Let me know how you can tell when your work with a given scene is done. Or tell me where you've found good examples of well-rounded stories.