How to avoid supervisors with prejudiced views? There seem to be professors who hold beliefs such as: 


Women students pose a special risk to male supervisors
Certain ethnic groups are smarter/lazier/harder-working than others
Older students are worse investments than younger students
Students with disabilities are less capable/more trouble than nondisabled students


No student should be saddled with negative prejudices from their supervisor. But how can this be prevented? 

The big problem is that bigots are often hard to identify early on. They think of themselves as just being honest and realistic. Asking their current students might not help, because if they don't belong to a marginalized group, they will often only notice other characteristics, such as that the supervisor is nice and explains things well. Unfortunately, it's possible to be a very nice guy and also a horrible bigot. 

There's also a corrosive secondary bigotry where the bigot insists that they themselves aren't bigots, but other people are, so good opportunities would be wasted on the marginalized student. 

How can a prospective student identify these people and avoid them?

Note: There are very strange answers and comments here. I just want to know how a student can find out if supervisor might hurt a student because of hidden bigoted beliefs. Especially if the student does not have a lot of time to get to know the supervisor, for example when choosing a graduate program. I hope this is clearer.
I will sound snarky with this answer, but have you tried talking to them?

Your question may ask well be asked as "How do I determine the values of another human being before I meet them?" And I would say that is not something you can truly learn and understand until you talk with them.

You may respond with, 'What if they lie?' And that's a very valid point. But I would tell you that all of human social interaction is this very problem, and that you will not gain the wisdom of how to interact with humans without interacting with them!