Simple. We'll "know" when Islamic radicals are wiped out.
Which conveniently neglects the fact that 100 years ago, terrorists weren't Islamic but anarchists. 100 years before that, they were the French. 100 years before that...? Hell, 20 years ago it was the friggin' Irish. At any rate, terrorism as a tactic just "is", and by that measure you can't "wipe out" terrorists.
You can, however, potentially "wipe out" Islamic radicalism...in the sense that you can change the conditions under which it thrives. But other terrorists will pop up in their place.