I knew that Booker T. Washington was half and half, as was Frederick Douglass. (Actually they may have been more than 50/50 because we don’t know about the lineage of their slave mothers, but their fathers were white.) W.E.B. Dubois was black only by virtue of social convention and the one drop rule. But I’ve never seen the point applied to MLK Jr. What was his heritage?
For those with slave ancestry, records are either very good (if the owner kept breeding records) or nearly non-existent (most cases as few did), the reason why mostly just suspected or was stuff of family folklore before DNA testing became widely available.
DNA testing isn't expensive anymore, $50 or so in most cases. Swab a cheek, screw it back in the provided tube, send it off and get your results a couple of weeks later.
I don't suggest it as a primary source, but definitely helpful when there are no authentic records available.
People are often shocked to find different DNA origin results from siblings of the same parents. They shouldn't be. Just as traits like eye or hair color may differ depending on which sperm and egg cells combined, so do the national origin markers. Of course, they are not going to vary greatly. One brother isn't going to have 80% Native American DNA, while the other shows 20%, but a 5-10% difference is normal.