How do we tell and know who is of Black African Descent when many Dravidians of India and Sri Lanka, the Negri?

How do we tell and know who is of Black African Descent when many Dravidians of India and Sri Lanka, the Negritoes of the Philippines and Indonesia, the Melanesians of Polynesia and New Guinea, and the Aborigines of Australia have black skin ? Would ancestry genetic testing be able to tell ?