Richard Clarke, former counterterrorism adviser to U.S. presidents Bill Clinton and George W. Bush, has made a career of investigating disaster warnings. The way he sees it, catastrophes can happen at any time, so why should decision makers ignore a Cassandra? Now a cybersecurity firm CEO, Clarke is an expert at figuring out who is a conspiracy theorist and who is a credible source. He explains his method through a few case studies—on the Bernie Madoff Ponzi scheme, the Fukushima nuclear plant meltdown, and others—from his new book, “Warnings: Finding Cassandras to Stop Catastrophes.”