I've tried using Gamepad Viewer for my NES controller, but only the A and B buttons show any activity while the Dpad, Start and Select buttons don't show any response even though the page itself shows activity in the remapping section. I've tried remapping and still no Dpad, Start and Select.

Are there any decent alternatives I can try instead? I'm using an original NES controller plugged into a Retroport USB adapter into a laptop running Windows 10.

I use open joystick display for controller visualization. I have no idea if that will work on your setup though. https://github.com/KernelZechs/open-joystick-display/blob/master/docs/getting-started.md

i used to use padpyght. works for all kinds of controllers and is highly customizable