The problem is that you often don't know until you actually start arguing with them that they are irrational or just confused and misled.

George H Smith has a pretty good essay about arguing with people to convert them to rationality, " Atheism and the Virtue of Reasonableness". For example, he advocates the "Presumption of Rationality" - you should always presume your adversary is rational until he demostrates otherwise. I don't know if the essay is on-line or not, I read it as the second chapter of "Atheism, Ayn Rand, and Other Heresies."