Input Device Tester 64 by Pastbytes (Released Dec. 24th, 2022) - Here is a really slick diagnostic tool for C64 Joysticks (with up to 3 buttons), Analog PC joysticks, Commodore 1351 mouse (in port 1), Paddles, Keyboard and the Retropad.  

Here's a link to the release announcement over at Lemon64.