As a good rule of thumb, "When I hear a story, when should I be especially suspicious?" If you hear a story and you think, "Wow, that would make a great movie!" That's when the "uh-oh" reaction should pop in a bit more, and you should start thinking more in terms of how the whole thing is maybe a bit of a mess.
First, I'll share one of my recent email declines: "This is such a fascinating subject, but I fear I've taken on too many blurb requests in the past few months and gave myself a self-reinforced pause. My apologies"