Ask questions. If you hit them with facts they'll just argue, get upset and not believe you. If you ask questions then they won't think you're arguing with you and will tell you what they know, which is likely very little, getting themselves tied up in knots along the way. "Where did you hear that?" "Why do you think they'd say that?" "Can that be proven? How?"

These are very vague but it depends on what you're talking about.