To determine authenticity and authorship.
The specifics of an investigation will depend on the nature of the document being studied (e.g., historical or personal) and the goals of the inquiry. Generally speaking, a document will be examined from three different aspects: historical, scientific, and stylistic.
Tips on how to authenticate a document by comparing handwriting.
It is virtually impossible to pinpoint the age of an undated document, but there may be clues to the era. If it is a printed piece - say a greeting card - the method of printing, address, and even the stamp may be important.
At the very least, materials and techniques must be consistent with place and time. Many forgeries are identified by the presence of materials that didn’t exist at the time alleged.
A detailed paper analysis will detect every shred of physical evidence concealed in a document.
Comparisons of style are essential for authentication. Penmanship, cultural phrasing, and form of address can help to identify both the era and author. Handwriting comparisons can authenticate a signature or manuscript.
In some cases, all of it taken together can’t solve a historical puzzle. For example, there are five known manuscript copies of the Gettysburg Address. Testing has proved that all are authentic Lincoln documents. But it will never prove, definitively, which of the five Lincoln actually read from at Gettysburg.