Nonfiction asks readers to follow explanations about the world, but an engaging style does not by itself establish a claim. When an assertion matters to your understanding, pause to see how the author supports it.
Distinguish a personal example, a reported observation and a broader body of evidence. Each can contribute something useful, but they do not answer the same questions. Notice whether the conclusion reaches beyond the material presented.
Use references and notes to locate the original context when possible. Check the date, scope and wording of the source. A cautious statement in a study can become much more confident when repeated in a popular account.
You do not need to investigate every sentence to read thoughtfully. Focus on central claims and consequential advice. Record what is supported, what is uncertain and what would require further checking before you rely on it outside the book.