I am not aware of Google, whether it be Google Now or the Searchbar on your homepage, actually speaking the word "listening." I apologize if I am taking your question to literally.
If you just want it to listen and show you that it is indeed working, all you have to do is touch the microphone. It will then open the Google App, make a noise and show the word "Listening...." From there you simply speak and it will do the search, start a timer or just about anything else you ask.
Are you running the stock rom? Cyanogenmod or OxygenOS? What version? Are you rooted? If you provide some more information, it should be a very quick fix.