A disciplined sort of triage can work for this. First, do they have real business experience? Ideally, have they started successful businesses before? A lot of the learning curve of startups is the learning curve of just being an entrepreneur.
Also, does their idea make intuitive sense to you, especially in terms of how can this be monetized? Vague, handwavy social apps that don't charge directly are a common problem. Apps for customers who don't have money is another popular antipattern. Apps that seem trivial are generally bad. Apps that respond Boolean true to "Can I already do this with Yelp?" are toxic.
But you find someone who has a couple of businesses already under their belt, who has some specific customers in mind who have a specific problem that person already understands... then you're getting somewhere. And it may be an entirely unsexy problem. One of my favorite startups locally is a company that has built a system for detecting excessive water use/leakage in toilets - something that can be repaired at cost savings if you have a large building (say, apartment complexes). Totally unsexy. Great market.