How can I work on which particular pages a CSS rule is used?
You can see a list of selectors not used on a page using external tools, there is no way for drupal itself to create the list for you however.

You can find some suggestions here: https://stackoverflow.com/questions/33242/how-can-i-find-unused-images-and-css-styles-in-a-website
I personally use the chrome developer tools as described in the above link.