If we try to skip the "pre" part and we limit ourselves to the "judice", the judgement afterwards, then I think that the most important criteria for judgement is actual behaviour.
It's possible I like or dislike a culture or religion in general. But the only thing that matters is actual behaviour from people.
So to come back to the prejudice that the Senegalese only want money or marriage from white people (Toubabs). Whether this prejudice is true or not with regard to a specific person I meet, I can see this from his behaviour. People to whom this prejudice applies, usually start asking me stuff immediately. "Hey Toubab, buy me a drink, give me this, give me that." They treat me like an object then, like a walking wallet. People who are not like that can be recognized because their first action is to give, to invite me to come and drink tea with them, to share their food with me. Good behaviour is a sign that a person has a good heart.
Labels like a name, a religion, a culture, a background, they don't say much about a person.