When it's appropriate to do so, and it's information that can be verified you can say, "Where did you get your information/Where did you learn that?" which may

When it's appropriate to do so, and it's information that can be verified you can say, "Where did you get your information/Where did you learn that?" which may be slightly less confrontational. If it's just an opinion, well, you might ask "how did you arrive at that conclusion?" or you might smile and say, "Oh, yeah?"