What is YOUR definition of 'extreme'?
The phrase “I know it when I see it” is a colloquial expression by which a speaker attempts to categorize an observable fact or event, although the category is subjective or lacks clearly defined parameters. The phrase was famously used in 1964 by United States Supreme Court Justice Potter Stewart to describe his threshold test for obscenity in Jacobellis v. Ohio.
Similarly, you will recognize religious extremism when you see it.