This program will allow you to determine the button/thumbstick mapping on your GamePad or Joystick.
Here is the GitHub repository for this program:
https://github.com/AldworthClass/GamePad-Detector
1 - Clone or Copy the code (and add the SpriteFont) to your program.
2 - Run the program and plug in your GamePad. Try out the buttons, it will tell you exactly which button maps to which input.
3 - The preferred input would to be to use GamePad, but if you have an unsupported 3rd party controller, you may need to use Joystick.
4 - If you are on a newer version of Monogame, you may beed to change the Target Framework in Project Properties