They are supposed to double check--see Gertz v. Robert Welch, Inc.. When a person who is not a public figure is defamed, the plaintiff only has to show that the defendant was negligent in making his defamatory statements, rather than the standard of recklessness to which he would be held in a case involving a public figure.

http://en.wikipedia.org/wiki/Gertz_v._Robert_Welch,_Inc.