Detekt is a free tool--the first of its kind--to check for surveilance software, the kind used by governments, on your computer. According to Amnesty International head of Military, Security, and Police, Marek Marczynski, "Detekt is a simple tool that will alert activists to such intrusions so they can take action. It represents a strike back against governments who are using information obtained through surveilance to arbitrarily detail, illegally arrest and even torture human rights defenders and journalists."
It's basically like a virus or malware scanner except it's designed to find spying software hidden on Windows. As with virus and malware scanners, even if Detekt doesn't find any spyware, that doesn't mean there isn't any. Although it's not a surefire way of seeing if surveilance software is installed on your computer, it's another tool to help you protect your privacy.