The NVIDIA SHIELD is the first Android TV box to receive the Google Assistant, and you can already do something cool, like using it with the TV turned off. If you wish to receive a visual notification you have heard, there is also a hidden setting.
There is actually a pair of flip-flops in the Developer Settings menu that causes the controller or SHIELD to flash when it detects the words "OK Google", which makes use much easier when you are not on TV. … assuming you have the controller nearby, of course.
Before you can activate this tweak, you must first enable the development options on your SHIELD. The process is exactly like it's on Android phones and tablets but here's the quick and dirty: skip to Settings, then scroll down to About. In this menu, find the "Build" entry, then click on it seven times. Toast notifications will let you know how many additional clicks you need, and on the last, it will tell you that "you are a developer." Completed.
Once done, go back to a menu and scroll down to the newly added Development Options entry. Open it.
There are a lot of things in this menu-a lot of things you probably do not need to worry about right now-so scroll all the way down to the bottom. Here you will find two options: "LED flashing controller when detecting keywords" and "SHIELD pulse LED when detecting keywords in first pass".
Activate one (or both!) With the rocker, and it's gone. Now, every time your SHIELD detects the keyword, it will give you a visual notification not only on the TV, but also on the controller and box if you wish. Neat.
Note: As these buttons are found in the Developer Settings menu, they may be removed with an update because they are not intended for everyday use, but only for the purpose of test. You have been warned.