How do we know that some people of African descent were not slaves in Spain's American colonies?

its the creoles for apex.

It is true that some people of African descent were not slaves in Spain's American colonies. We know this because of the written records that have been left from that era. Although most African people in America were enslaved, some of them were able to achieve their freedom, particularly in the Spanish colonies. There were several pathways for this. Some slaves were emancipated. Other had such a small percentage of African heritage that they could be considered members of other races. Finally, some others were able to buy their freedom or achieve it through heroic and admirable deeds.

Statement first is true in this regard because the Spanish had a caste system that included free Africans and former slaves. Cristopher Columbus established the "first Spanish colony" in America, in what now is Haiti and The Dominic Republic.

Since the beginning, Spanish used slavery from Indigenous people in the Americas and they also brought black Africans. Spanish had "Castas", the equivalent of racial and social hierarchies. Spanish considered slaves inferior beings.