Responsive images
External resources
Image Tag tester
Bookmark image tester for any site
info
if you generate with density option like 1.5 or 2 and a breakpoint is also near that breakpoint, the tool will complain about similar image sizes and 2 different resources, but the images are usually in different quality settings.
https://ausi.github.io/respimagelint/
Web.dev - Lighthouse online
Lighthouse is a testtool for pages including tests for modern formats and properly sized images in Chromium based browser or online.