How and/or when do you know an idea isn't working?
That's a tough question, really. I usually have ideas, write them down, mull over them. Sometimes I'll start an outline, or maybe write a couple pages of notes on what the story is about. When that process doesn't work...if I lose interest before the first pages are written, I know I can't write that story.
Another test of durability is making it past the fifty page mark. That seems to be where I always clam up and grind to a hault. If I can outline, brainstorm, or just push my way through I know it'll go the distance. Otherwise? Well...it has yet to happen yet. But I suppose it'd go to the junk yard.
What about you? How do you know when an idea won't work?