The same way you go about finding out if they're good teachers, or if they're martially capable, or if they're religiously tolerant, or if they're racially tolerant, or if they're decent people in general, etc. You ask around, you train, you get to know them. The problem in my eyes is that you're asking about one particular type of intolerance in a world where there are all sorts of types. If one is to post a sign saying "lesbian friendly" shouldn't one also post a sign that says "women friendly". And "Men friendly". And "Gay Friendly". And "Hispanic Friendly". And "Islam friendly". And it goes on virtually ad infinitum.