External criticism is a process by which historians determine whether a source is authentic by checking the validity of the source. Internal criticism looks at the reliability of an authenticated source after it has been subjected to external criticism.
To authenticate a source, historians examine whether or not the language conforms to language used during the period. They also investigate to see if the purported author exhibits ignorance of things that someone during that time would know or, conversely, demonstrates knowledge of things not yet known at the time. External and internal criticism provide the basic methodology of modern historical research.