@mitu Thank you!

Maybe this is your solution https://www.linux-projects.org/uv4l/tutorials/play-retropie-in-browser/

I tried to install it yesterday, but I run into some problems I havent figured out yet.... But I've seen people successfully installing this solution.

Thanks for the info.
I knew jstest-gtk. I didn't know it is available via console, too.
I'm gonna test it with a USB gamepad first, to check if it's what I need (I suppose so). NES/SNES controllers test will take a while, since I need to prepare the interconnection board. Maybe I will do a quick test with jumpers.