You Android device can detect your voice and whenever you say "Google" it can open and launch Voice search and let you enjoy the benefits of Google Now.To enable voice detection on your Galaxy device, do as follows:
1. Open Google App (the app's icon is Blue)
2. Press Menu Button and then go to Settings
3. Tap Voice
4. Make sure "Hotword Detection" is checked.
See Also: How to Open Google Now on Nexus 4