How do you know the author is presenting factual information?

The book I am in the middle of right now is a non-fiction book about a confidential computer break-in that happened some years ago. Clifford Stoll is the author of this book is one of the actual characters of this book so he narrates it himself. And to answer the question, I know that this story is true because of 2 very obvious(to me) reasons.

1.The book is labeled NON-FICTION.
2.There is an old article about the actual break-in in the back of the book.