This question does not have a simple answer. The answer depends on the time and place, the context in which enslaved people were held in bondage. By and large, masters know quite a bit about the ethnicity of "saltwater slaves," or African bondspersons who survived the Middle Passage and made it to North America. They often selected specific ethnicities that had reputations for docility or compliance. For instance, the Bambara people, a Mandé group from the Senegambia, was particularly known for supposed compliance. Other West African groups were selected for their expertise with certain cash crops, like rice or indigo. My expertise is in 18th-century Louisiana, so my examples will draw from this location.
While is seems that specific ethnic knowledge seems to diminish after the British Empire ended the slave trade in 1807, we have substantial evidence that masters were aware of the ethnicities prior to this point. One of our best sources for this is the Records of the Louisiana Superior Council. if you decide to peruse these documents, you will see that court officials would include ethnicity any time an enslaved person ended up a subject of court inquiry (rebellion, arrest, last wills, etc.). Louisiana itself saw two major slave conspiracies that were organized primarily by specific ethnic groups: first in 1731 with the Samba Rebellion, second in Mina Conspiracy in 1791. Both were exposed before they could be executed, but were planned within specifically ethnic lines (Bambara and Mina respectively). Subsequent court cases relevant to these cases make it clear that both the plantation masters and the courts were aware of their ethnicity prior to the events.
Sources:
In the case of the French Caribbean (Saint-Domingue, Guadeloupe, Martinique), not only the slave business sector (owners, traders, administrators) was aware of the ethnic origin of the people they bought or sold, but that information was considered extremely important for determining the economic and work value of an enslaved person, along with their age, sex, and alleged or potential capabilities. One of the most famous texts about colonial-era Saint-Domingue, Moreau de Saint-Méry's Description topographique, physique, civile, politique et historique de la partie française de l’isle Saint-Domingue (1797), dedicates an entire chapter to the description of more than 30 African ethnic groups. He provides for each group - often called "nations" in other texts - a description of its physical and moral characteristics, an assessment of its suitability in the slave economy, and some useful tips for the successful slaveowner^(*). About the Ibos:
The negroes of the Gold Coast are esteemed for cultivation, but in general their haughty character makes them difficult to manage, and they require masters who know how to study them. It is chiefly with regard to the Ibos that a great deal of supervision is necessary, since the slightest grief or discontent leads them to suicide, the idea of which, far from frightening them, seems to be somehow attractive to them, because they adopt the dogma of the transmigration of souls. The Ibos in a plantation have all too often been seen to form the project of hanging themselves to return to their country. When it has not been possible to completely prevent this Pythagorean journey, the head of the first man to kill himself is cut off, or only his nose and ears are kept on top of a pole; then the others, convinced that he will never dare to return to his native land, thus dishonoured in the opinion of his compatriots, and fearing the same treatment, give up this dreadful plan of emigration. This disposition of the soul, which makes the Ibos referred to by these Creole words: Ibos pend' cor à yo (Ibos hang themselves), makes many colonists afraid to buy them.
For Saint-Méry, the Franc-Congos have a "a sweetness and cheerfulness which makes them sought after", they are intelligent, and they love singing and dancing. They are thus valued as domestics, contrary to the Arada women, who are "cantankerous and quarrelsome", "eternally chatty" but who cannot speak French. Those women have also a clitoris so large that "it could in some way fill the role of the other sex". The Mouzombés and the Mondongues have the "most hideous character" and are natural cannibals. And so on.
These ethnic stereotypes, which can be found in many documents of the same period (see Ducoeurjoly, 1802, for another example of ethnicity-based essentialism in Saint-Domingue), were much valued by slaveowners. Just like a modern farmer chooses the plant cultivar or the animal breed most suitable to their economic goals, the slaveowner preferred slaves from a certain "nation" over another for a specific type of work, depending on their alleged character, physical appearance (height, physical strength...), hardiness, resistance to diseases, tractability, "natural" skills etc. Ethnicity would decide (in part) of the occupation of the slave: field workers in coffee or sugar plantations, sugar boilers, livestock herders, specialist craftsmen, domestics etc. This was reflected in the market value of the slaves (see King, 2001 for examples corrected for age and gender).
The (supposed) "nation" of each slave was recorded in notarial acts and plantation registers, which has helped historians (after the pioneering work of Gabriel Debien in the 1970s) to study the demographics and social history of slave populations at plantation and regional level. A thorough analysis of the ethnic makeup in the slave labor force of Saint-Domingue has been carried out notably by David Geggus (1993). For instance:
Some nations had greater success in avoiding field work than others. This was especially true of the tall peoples from the savanna regions. With the exception of the Fulbe, they were disproportionately selected as sugar boilers, which was by far the commonest post available outside of field labor. They appear to have had an aptitude for standing long hours over steaming cauldrons. 32 The following proportions of the men were employed as sucriers: Hausa, 57 percent; Bambara, 25 percent; Nupe, 23 percent; Mandingues, 22 percent; Africans in general, 16 percent. However, 25 percent of both Igbo and Yoruba worked as sugar boilers, although they were only of average height. While the prominence of Yoruba as sucriers is hard to explain, 33 the large percentage of Igbo specialists on both coffee and sugar plantations may have been due to their poor reputation as field workers. Such reasoning may also have been applied to the "slow-moving" Bambaras. The planters' avoidance of those deemed the ablest agriculturalists, the Aradas, also points to this conclusion.
We should note that those ethnic categories are in fact extremely debatable and may not fully reflect the reality of the ethnic makeup of the enslaved populations. Stewart R. King (2001) remarks for instance that the Minas, a small coastal group in Togo, seems overrepresented in notarial statistics, while their Ewes neighbours, a much larger one, is absent: "Mina" may have simply meant "sold to Europeans by the Minas or at Mina castle". The Aradas, mentioned above, may have come from any of the 70 ethnic groups that entered the slave trade from the coast of modern Togo and Benin. Some "nation" names that appear in the record seem to have been just village names, or even people names. This would have made the reliance on ethnicities for slave management not that useful, but it still played a notable part in the decision-making of slaveowners.
^(*) Saint-Méry, like many Enlightenement intellectuals, was fond of categorization: he also described about a hundred combinations of White, Black, and Native Caribbean people, defined by their amount of white or black "blood".
Sources
Debien, G., and J. Houdaille. ‘Les Origines Africaines Des Esclaves Des Antilles Françaises’. Caribbean Studies 10, no. 2 (1970): 5–29. https://www.jstor.org/stable/25612210
Ducoeurjoly, S. J. Manuel des habitans de Saint-Domingue, contenant un précis de l’histoire de cette île, depuis sa découverte. Tome 1. chez Lenoir. A Paris, 1802. http://www.manioc.org/patrimon/SCH13063.
Foubert, Bernard. ‘L’origine des esclaves des habitations Laborde’. In L’esclave et Les Plantations : De l’établissement de La Servitude à Son Abolition. Hommage à Pierre Pluchon, edited by Philippe Hroděj, 103–23. Histoire. Rennes: Presses universitaires de Rennes, 2019. http://books.openedition.org/pur/97658.
Geggus, David. ‘Sugar and Coffee Cultivation in Saint Domingue and the Shaping of the Slave Labor Force’. In Cultivation and Culture. Labor and Shaping of Slave Life in the Americas, edited by Ira Berlin and Philip D. Morgan, 73–98. Charlottesville and London: University Press of Virginia, 1993. https://books.google.fr/books?id=QpyWi_NCyukC&pg=PA73.
King, Stewart R. Blue Coat Or Powdered Wig: Free People of Color in Pre-Revolutionary Saint Domingue. University of Georgia Press, 2001. https://books.google.fr/books/about/Blue_Coat_Or_Powdered_Wig.html?id=JX_SbvLE2_YC.
Moreau de Saint-Méry, Louis-Élie. Description topographique, physique, civile, politique et historique de la partie française de l’isle Saint-Domingue. Philadelphie: Chez l’auteur, 1797. https://gallica.bnf.fr/ark:/12148/bpt6k111179t.