On 11 May 2007 a Guilty Verdict was handed down in a U.S. - China Spy Case

This is a troublesome case in that it may be indicative of how difficult it may be to nab spies. After a six-week trial a federal jury convicted a Chinese-born engineer of conspiring to export U.S. defense technology to China, including information on an electronic propulsion system that would make submarines virtually undetectable.

Chi Mak exploited his low-key lifestyle and his good reputation to hide his real intent: conspiring to pass U.S. secrets to China for over two decades. Mak is a naturalized U.S. citizen who confessed to the crime and revealed that he lied to investigators.

This individual apparently had no warning signs that he was suspect until he finally was found out and revealed his real intent on his own.

How you combat a clever individual like this without infringing upon individual liberties is an open question.