There's never a way to be certain in these situations. If a minority is fired due to racism, unless their boss is firing with overt racist statements you aren't going to be getting solid proof.

How many professional interviews have you been in where they told you all about why they weren't hiring you, who they were hiring in place of you, and why? He found this information out after he became her boss and had access to her information.

This doesn't make it certain, but it's not absurd to believe that such a policy like affirmative action could have negative consequences. That's why quotas were outlawed by the supreme court. Why several states have decided not to use it.

The net benefit is positive, but it can very easily lead to negative situations. But say the work place was primarily white men, that is not necessarily indicative of anything. Those white men may have been more qualified than any other applicants, and so then to further deny a well qualified applicant because of race is morally wrong, but it must be done out of fear of legal consequences.