This differs from just knowing facts. It also involves an appreciation of what these facts mean to people's lives. For example, since we demand that others try to be informed when they deliberate about how to act toward us, we will, if consistent, demand this of ourselves too. So we'll hold the general principle that people ought to be informed when making moral judgments. These same rationality conditions also apply to desires.
Irrational desires have flaws like inconsistency, ignorance, or lack of imagination.
Accordingly, racist desires are irrational if they're based on social conditioning and would disappear if we broadened our knowledge and experience. Make sure that the racist has a clear understanding of the facts. Have him imagine himself, vividly and accurately, in the place of his victims. See if he treats his victims only as he's willing to be treated in the same situation. The racist will likely fail the GR test.