In general, the closer a source is to the event which it purports to describe, the more one can trust it to give an accurate description of what really happened.
In other cases, Bible scholars use the way a text is written (changes in style, vocabulary, repetitions, and the like) to determine what sources may have been used by a biblical author.
The documentary hypothesis considers the sources for the Pentateuch (the first five books of the Bible), claiming that it derives from four separate sources: the Yahwist, Elohist, Deuteronomist, and Priestly:[2] For example, of the two creation stories at the start of Genesis, the first is ascribed to P, while the second (the creation of Adam and Eve in chapter 2) is ascribed to J.
An example of this kind of source criticism is found in the book of Ezra–Nehemiah (typically treated by biblical scholars as one book) where scholars identify four types of source material: letters to and from Persian officials, lists of things, the Ezra memoir (where Ezra speaks in first person), and the Nehemiah memoir (where Nehemiah speaks in first person).
Source criticism also leads many scholars towards redaction of the book of Isaiah from original multiple authorship.