How to avoid supervisors with prejudiced views?
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!