The question is a rhetorical barb, an accusation that the hearer is deficient in his thinking. Presumably, it is an attempt to jolt the hearer into questioning his beliefs about race. Since the question is a rhetorical/motivational device, we should not be surprised that its claim is imprecise and, perhaps, even metaphorical.
The substantive gist of the barb is an implicit claim that race is a much deeper and more complicated matter than the hearer has supposed. The hearer thinks he knows race when he sees it, but the speaker thinks the hearer possesses only a superficial understanding of race.
We can only speculate on what the speaker thinks are the “deeper” truths about race. The possibilities include:
Genetics: there is no clear definition of race, and what might be taken as defining features are shared across groups in ways that make any precise definition arbitrary (i.e., there is no pure race).
Phenotypes: traits taken as characterizing members of a given race appear in gradations among all supposed races, so that some people are not clearly members or non-members of a given race.
Anthropology: there is no set of social, cultural, or technological practices that clearly and unambiguously defines a race.
However, the “deeper” truths held in mind by the speaker may be experiential rather than biological. Here, the accusation would be that the hearer thinks he knows what it is like to be a member of a given race in a particular society, but the hearer has only a very incomplete understanding.
It is obviously true that the hearer, the speaker, and everyone else, has only a very incomplete understanding of how others experience life. It is the human condition that we will never have more than a fragmentary awareness of the kinds of lives lived by the billions of other people on earth. This is true to the point that a member of a race, however defined, will have only a fragmentary knowledge of the experiences of the millions of other members of his own race. The original accusation, however, rests on the assumption that the hearer has a self-satisfied opinion of himself as knowing and understanding either what defines race or what it is like to be a member of another race.
While the above possible considerations by the speaker highlight our common human ignorance, it is possible the speaker believes there is knowledge to be had about racial experience. The speaker may believe that experiences tend to differ in kind according to race in a given society, but that the hearer is oblivious to that difference. In that case, the accusation is that the hearer thinks he has a fairly complete understanding race but, in fact, lacks an understanding of what is like to live as a member of different race. The accusation would then be a tool to prod the hearer to listen to the experiences of others, and, so, to acquire a better understanding of his society, the people in it, and his role in sustaining such a society.