1) Those Gosho still extant in Nichiren Shonin's (goshinseki) handwriting are clearly authentic.
2) Those goshinseki destroyed in the Minobu fire of 1876' No one questions most of these works, e.g. the Kaimoku sho. however, one of these, the Shuju Ofurumai gosho, a partially autobiographical work, is still debated. Nevertheless, since a very early Fuji-ha biography of the Patriarch cites two key passages from this work, it seems likely to be genuine, for the original copy was at Minobusan.
3) Copies by Nikko and other first generation disciples. Also a few copies by slightly later disciples
4) Works with no early copies but doctrinally and stylistically close to the above works
5) Works that have no early copies and have doctrinal and stylistic differences with the above 1-4 categories. These are clearly dubious.
The San Dai Hi Ho Sho is a widely debated work with some relying on a computer study of its vocabulary to assert its genuineness but it is clearly close to the Fuji-ha. Nikko does not mention it when discussing the Three Great Secret Laws.
The Shoho jisso sho, in the opinion of many. is a forgery in its first two or three pages (STN, v. 1, 723-725) which was supposedly written a short time after the Kanjin honzon sho. These pages expound a form of medieval Tendai original emlightenment [hongaku] without mentioning "hongaku" as such: It asserts the bombu or unenlightened worldling as the real Buddha. Thereafter, however, it returns to a more conventional view that is close to the genuine works of Nichiren Shonin (classes 1-3 above); on the other hand even parts of the more authentic-looking part (page 725 on) appear to be pasted together by a later copyist and there are different modern versions of this section in part. I still think the part from p. 725 on maybe a fusion of two or more real letters of the Patriarch.
The Issho jobutsu sho has some genuine-looking passages but also some slightly hongaku passages; this might be explained as the remnants of Nichiren Shonin's Tendai training and so scholars date it to 1255 (relatively early). On the other hand its advanced exposition of the Daimoku faith points to a time after 1260. These facts lead me to be very suspicious.
Please note that some of the dubious works attributed to Nichiren Shonin contain passages that summarize genuine ideas of the Patriarch BUT we must be aware that they do not have the same certainty found in authentic works.
I hope this is useful.