How would a would-be novelist know they’ve found the right idea?
The right idea is one that obsesses you. Don’t choose an idea because it’s fashionable or because you think it’ll suit the market. There are two reasons for this. First, tastes change. What is cool now may be tired-out in a few years. Second, for the many, many, hours that you sit writing, it is only you and the story. You have to be in this to delight yourself. You’re going to be working on this for years, so choose an idea that will keep you interested.