I've struggled with this as well. As far as I can tell there isn't a good solution, but I'd love to be proven wrong.

You can run PurgeCSS in a dry mode to find any unused selectors, but it can't handle any classes that are dynamically generated. If your codebase has used anything like `buttonClass = 'button-' + color`, you may get false positives.

The Chrome Dev Tools coverage can show you how much of a stylesheet is used at runtime, but if you concat everything into a single css file that's used for every page, a single page's usage doesn't help.