Probably the best rule is trust nobody until they prove otherwise. From Robert Parry at strategic-culture.org:
The looming threat of World War III, a potential extermination event for the human species, is made more likely because the world’s public can’t count on supposedly objective experts to ascertain and evaluate facts. Instead, careerism is the order of the day among journalists, intelligence analysts and international monitors – meaning that almost no one who might normally be relied on to tell the truth can be trusted.
