If you’re writing nonfiction, you may want to invest in the services of a fact-checker. Fact-checkers adhere to a rigorous standard, questioning assertions and asking for documentation and citations to support those assertions.
The self-publishing service Lulu has some good tips on fact-checking here. Probably the greatest portrayal of a fact-checking department was written by John McPhee about The New Yorker.
Magazines employ fact-checkers because their publication cycles are not as severe as newspapers, and magazine articles are shorter than book-length manuscripts and therefore not insurmountable for a staffer to fact-check.
Newspapers and book publishers generally don’t have fact-checking departments – for completely different reasons. At newspapers, the reporter is the source of the facts, and it’s up to him to maintain accuracy to the best of his ability (and report corrections later, if anything needs to be corrected). The deadlines for news publications are severe (even more so in the age of the digital newspaper).
At book publishers, the quantity of information being published is so great that holding up publication of any given book is too onerous a burden. Again, the writer is responsible for the facts in the book. And frequently, the writer must either be that fact checker himself, or contract that work out.
If your book discusses the lives or actions of people who can be identified, you may want to have it vetted by a lawyer. This will help avoid potential lawsuits – defamation, libel, or other charges. The self-publishing service iUniverse offers a great checklist to help you determine whether you need a lawyer to vet your manuscript.