Questioned document examination (QDE) is the forensic science discipline pertaining to documents that are (or may be) in dispute in a court of law. The primary purpose of questioned/forensic document examination is to answer questions about a disputed document using a variety of scientific processes and methods.
Many QD examinations involve a comparison of the questioned document, or components of the document, to a set of known standards.
The Wikipedia page  has a general overview and examples etc.
ASTM  has published a standard guide for the examination of handwriting entitled "E2290-07a: Examination of Handwritten Items".