Ask HN: How do you tell whether there's a path out of an idea?

"How do you tell whether there's a path out of an idea? How do you tell whether something is the germ of a giant company, or just a niche product? Often you can't.... sometimes I can see a path that's not immediately obvious; that's one of our specialties at YC."

=> Are there good alternatives to YC (a privilege not everyone can get), to get a 2nd-opinion validation of whether a startup idea is a path to a bigger one?