Often, where the question as to an addition to a document has been raised, a determination as to the composition of the ink in the questioned area is one of the examinations undertaken. As an example, if an all of the entries on a record keeping page (such as nurse’s progress notes) are written in one ink and a single entry is found to be a different ink, that entry may have been written into the record at a subsequent time. Instrumental analysis such as video-spectral comparison (infrared) or thin layer chromatography (chemically) are two common methods to differentiate between inks.
A more comprehensive discussion of this process can be found in here.