Hamon
According to the Japanese Wikipedia, his work is like that of Ikkanshi Tadatsuna: almost always in nie, with an ō-notare and a midare in the manner of tōran-ba (濤瀾刃, tōran-ba); rarely, a suguha with a deep, bright and lively nioiguchi.
What belongs to the school
This is how the Japanese Wikipedia describes the school of Tadatsuna, not Tadayuki: ko-itame with mokume and fine ji-nie, masame on the shinogi-ji; a temper with the straight Osaka start, the 大坂焼出し (Ōsaka yakidashi), wide, and a rhythmic gunome; a straight bōshi, ko-maru with a short turn-back.
Nakago and mei
The first signed 摂津守藤原忠行 (Settsu no Kami Fujiwara Tadayuki), and before the title, 摂州住藤原忠行 (Sesshū jū Fujiwara Tadayuki); those who followed, 摂津守源忠行 (Settsu no Kami Minamoto Tadayuki), according to the Japanese Wikipedia. The same source says that the signature of the first is very similar to that of Tadatsuna.
What is not on record
None of the sources consulted describes the sugata, the jihada, the bōshi, the shape of the tang or the file marks peculiar to Tadayuki, nor how his generations are told apart.