You engage in conversation in such a way as they come to the conclusion on their own without having to steer them.Then when they tell you,you acknowledge their brilliance and thank them for opening your eyes! There is a little sarcasm there but I mostly mean it.You direct your questions to them in such a way that they can come to the conclusions that you know to be true. Most people have a problem with letting go of preconceived notions,and if it seems like their notions are being challenged they will defend them,whether they are right or wrong.
It takes huge amounts of patience and tolerance some times to do this.I see it professionally all to often.When you have people who have no idea what they are talking about and defending what isn't so.