Kenny's Gamepad Reader can read key codes, buttons, and joystick axes for any gamepad!
Free - Kenny's Gamepad Reader for WebGL
Press any key (or gamepad button or axis)
You can see above how Unity would read the input in WebGL.
Free - Kenny's Gamepad Reader on Google Play
Gamepads may read differently on different target platforms. For example, an xinput gamepad on WebGL may read the d-pad as buttons, but the same controller on PC may read the d-pad as joystick axes.
The Google Play app will show you (for free) how gamepads read on Android.
$2 or more - Kenny's Gamepad Reader for Windows, Mac, and Linux
With all the apps, you'll see how gamepads read on PC, Mac, Linux, Android, and WebGL.
The $2 tier also includes an Android .apk that you can install on Fire TV!
In order to download this tool you must purchase it at or above the minimum price of $2.00 USD. You will get access to the following files: