I have faced a problem on my clients task . Client asked me to find out which images are responsive or not responsive on the web site.
Well the best thing to do would be to resize the browser window and see if the image size scales with the browser size getting smaller or bigger... If that doesn't work try poking them with a stick and see if they respond.
Many online tools out there to check image optimization and page responsiveness, you can go and check there. If you have wordpress website, use this plugin https://wordpress.org/plugins/ricg-responsive-images/