Although I may be accused of taking too simplistic an approach to this question, I really do think that it comes down to just one thing.

Well, in practice, of course, there are many potential indications of success. For example, people telling you they like what you write. People asking when your next book is coming out. That sort of thing.

However, nice as such accolades are, they don’t pay the rent, and talk is, or can be, cheap.