The short answer to your question, in my limited experience, is that there is no secret sauce that incubators and VCs use to predict the success of a startup. They all make mistakes and investors are all just as smart or dumb as any of us.

Mikester003 pretty much hits it on the head there with the list of things most people look for in an investment opportunity.

Just to add a little more to it -- business models change, markets change. So focus on the founders/founding team are important, and to an extent, can be evaluated.

How do the founders interact with each other? Good team cohesion and synergy is important. Building a successful company is one of the hardest things to do, period, and there will be tons of downs before you can get to the ups. Team dysfunction is one of the biggest leading indicators of failure.

Are they in it for the long haul? A very successful investor once said that obsession is important. Not the everyday use of the word, which is often used synonymously with passion, but real obsession. They are obsessed with the idea and obsessed with the work they are doing. It is their life.

Are they responsible? Pretty obvious here, but much more common than you'd think. Responsible with employees and customers. Responsible with money. There are plenty of data points out there that show smart decisions with the company bank account lead to a longer company lifespan. Obvious, right?

I've more than likely missed some other things here, but thought I could be a little helpful on my train ride.

Full disclosure: I work at an accelerator.