You're not supposed to. It's a covert act. Watergate, for example, was a classic case of espionage incompetence in action. This apparent quiet gives the victim a false sense of security. Not only is information theft prevalent and invisible, it is also silent.
Discovery relies heavily on the victim's intuition and preparedness to handle the problem.
Due to the covert nature of spying, we will never know for certain. Fortunately, however, we can use the failed espionage attempts as a gauge. They reveal over and over again that the problem does exist. Word filtering back through the press and from electronic eavesdropping detection specialists can make you a believer. It's happening on a daily basis.