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

One way we know that some people of African descent were not slaves in Spain's American colonies is because the Spanish had a caste system that included free Africans and former slaves," even thought this was not as divided as it sounds.

During the time of the colonies, Dutch traders brought African slaves taken from a Spanish ship to Jamestown; in North America, the Africans were also generally treated as indentured servants in the early colonial era-

Answer: Because the Spanish had a caste system that included free Africans and former slaves