How do family historians know they are producing or receiving trustworthy results? 

This official manual from the Board of Certification for Genealogists, essentially a users' guide for family historians, provides standards for genealogical researchers to assess their own and others' work. The revised second edition, published in 2019 with additional revisions in 2021, increases the clarity of DNA and privacy standards. Those standards are especially useful in the twenty-first century, when many genealogists use a complex new tool—DNA testing—and trace living people more often than they did in the past.