To determine if a signature or document is genuine requires the examiner have knowledge of the style of writing that is on the document.  The signature may be written in script or print script (printed). The document may be typed on a machine or a computer.  Computer printers are varied and use different types of printing.  Typewriters (not frequently used any more) may well have erasures or corrections.  Though computers have spell check today, they do not correct according to grammar and structure of sentence's.
Many tools are used to determine the genuineness of the document or signature. Older documents may be of the typewriter genre.  The examiner will use the entire lab for studying documents.
Most document examiners have, until relatively recently, been taught by the military/government.  Now Document Examination and Analysis are learned by a course given by private institutions, (such as Handwriting University.) They are available on the Web.  It is intensive instruction and practice time.   Call me NOW to answer your questions about documents and their alteration/forgeries.