[KevinH]

Try the wrCisco's cssUndefinedClasses plugin for Sigil.

Or as I explained earlier, merge the html classes report with the css selectors used report. You can even do that manually by just checking what few classes are not found in the html classes report (sort on that column) with the css selectors report of found combinator selectors.

But even that will not tell you if a class was added to help identify an element for some other reason such as javascript code, etc.

Alternatively you can use Search to find any not found class selectors used in the xhtml file and use Preview's Inspector to see the fully inherited CSS of that element.