I have a personal mental checklist that I use to evaluate the success probability of a new venture: can it move from the planning stage to a financially stable self-supporting business. Some science here, some "gut feel", some "told you so" experience coming through ... but here are some of my "red flags": things startup team members say and what I really "hear".
They say: "We don't have any competition!" ... I hear: "We haven't done our homework to see what else is really out there."
They say: "We the first to ever do this!" ... I hear: "So who cares? If it's never been done before, maybe that's because there's really no need to do it."
They say: "Everyone is going to want one of our new gizmogadgets!" ... I hear: "Our family and close friends think our gizmogadget is really neat!"
They say: "We're going to build a much, much better gizmogadget and sell it for half as much as the competition!" ... I hear: "We have no idea how much it will really cost to make our gizmogadget or how much it will really cost to market it."
... and there are more! Check back every now and then, and I'll update this post.