There are several places where we can find evidence of prejudice. In particular, Juror Ten is biased. Juror Four is also biased, but it is more subtle.
Here is what Juror Ten says:
We don't owe him a thing. He got a fair trial, didn't he? You know what that trial cost? ....Look, we're all grown-ups here. You're not going to tell us that we're supposed to believe him, knowing what he is. I've lived among 'em all my life. You can't believe a word they say.
Notice the language here. This juror groups everyone in the slums as "them." Then he goes on to say that you can't trust a word "they" say. So, he does not see poor people as individuals, but as a group of degenerates. Here is an even more charged statement from the same juror:
"I don't understand...Look, you know how those people lie...They don't know what the truth is....Human life don't mean as much to them as it does to us...Look, these people are drinking and fighting all the time, and if somebody gets killed....They don't care."
Juror Four also shows signs of prejudice. Though this juror tries to be more tactful, the underlying assumption is the same.
"The children who come out of slum backgrounds are potential menaces to society."