The truth is that Indians should know better than this. If we booted everyone who wasn't a "fullblood" off the continent, there wouldn't be enough Indians left to have one nation, let alone five hundred. Non-Indians should also know better, but you don't want to get me started on that. After five hundred years of intermarriage, what does an Indian "look" like? And do we really want to get into race theories about who's "full-blood" and who's "mixed-blood?" Even to mention those words implies that there is some sort of racial purity thing going on. I only call myself a "mixed-blood" because it's a handy way of placing me in the Indian community. I certainly don't mean that my blood is less "pure." And what is the scientific test for measuring Indian "blood?" Is your blood B positive, AB negative, or Indian?
My point is this. Whoever is challenging you, probably doesn't know anything about you, your family, or your ancestors. So, how do they know who you are, or are not? Don't take their word for it! Go out and prove it to yourself.