There are certain approaches that can aid one: Is it in Nichiren's hand, signed and stamped?; are they in one or the other of the earliest collections of Gosho?; congruence with the content and principles espoused with respect to the period in which Nichiren taught [the stage of development of Nichiren's teachings]; are there citations, principles, or events in the Gosho from a time after Nichiren's death?; comparing content of Gosho to the Five Major Works and the entire body of authenticated [in Nichiren's hand] works; computer or reasoned analysis of writing style and the words contained in the Gosho [again, comparing Gosho written in Nichiren's hand to the Gosho in question]; and the approach of textual parsimony [limiting ourselves to Gosho in Nichiren's hand and weighing Gosho according to profundity and the preponderance of established concepts.
A lesser criteria: That which the vast majority of priests and scholars have deemed authentic or one particularly learned priest or scholar.