I use a simple metric:

How will my autobiography read?

If I'm improving the quality of my autobiography every day, then I'm probably doing something right. If I'd be too embarrassed to write down some aspect of my life, or if it would have too tenuous a connection to the rest of the story, then I probably need to change course.

This metric also makes it clear that success is in how you live your life rather than where you end up. Your life's final destination is only going to fill one page. The journey of your life needs to fill the book.