Ok, by your signature you seem to be studying Ronald Duncan's system. I'm a bit hazy on my knowledge about him, I have to admit, but if I remember correctly, he claims to be a Koga ryu ninjutsu adept. Where did he learn Koga ryu ninjutsu? Who taught him? And who taught his teacher? etc. I think you get the picture... Oh and do remember, there are people who know the history of ninjutsu quite well indeed (I'm not one of them)
I'm a little hazy myself, as I am not a direct student. I can also remember a time when the Koga term was used in magazines but I never hear of it in my classes. We don't wear tabi or ninja garb, and for the most part practice a striking/pinning/throwing art. From the website, he seems to be attributed to "American Ninjutsu" so perhaps it is his own thing.
I do know that he has an extensive background in many arts, spanning 40 + years, and so has merged them together into his own system, and he is very open about that, so in my book, he is not a fraud.