The basic method of handwriting identification is the same now as it was in Twelfth Night: to compare the questioned writing with other writings by the supposed writer. This can be done from memory if (like Malvolio) one is already familiar with the claimed author's handwriting, or by examining the questioned document together with known samples. It's a simple, obvious task. Any person-certainly any literate person--can have a go at it. The claim by handwriting experts, now and in the past, is equally simple: We can do it better.
Gross, Samuel R. "Detection of Deception: The Case of Handwriting Expertise." Va. L. Rev. 87, no. 8 (2001): 1847-55.