How Can We Know That an Action is Based on a Bias?

How can we know that an action is based on a bias? For example, how can we know that an action is based on a bias towards a group of people? For example, if Asian American students got more attention from the teachers (Lee et al., 2015), how can we know that the teachers gave more attention to these students because of their race? Before a teacher first meets an Asian American student in person, the teacher might have a bias based on the student’s Asian name. But after the teacher meets the student in person, if the teacher gives this student more attention, how can we know that this teacher’s bias is based on the student’s race? In theory, it’s possible that this teacher gives this student more attention, only because this teacher thinks this student to be smart. How can we exclude this possibility? There is no way to exclude this possibility.