This is a virtue that cannot be taught it can only be given the room and time needed to grow on its own. For discrimination (in this case) is the ability to be able to discern the truth in others. To see the real meaning of what people say. In other words to not be gullible.
As parents there is not much we can do in this regard. But we can help to point out our children's errors in logic. To show them, through example, how to see the real truth behind people. This requires us, as parents, to be candid with our kids.
Yet many parents I have met cannot have a real conversation with their kids. It is something that is beyond their capabilities as parents. But this isn't just in their role as parents they have a real problem having a "real" conversation with anyone. I talked about this in my post on efficiency as a virtue.
I encourage you to read that post and the book that it talks about. Then sit down and talk with your kids. They need and want that attention from you. Have regular conversations with them, it is the only way they are going to learn anything worthwhile.