I know about the option "Do not show unused categories" but it would be great if there would be a function to highlight or mark them. This would improve the management of categories and help the user delete what's not necessary.
I am closing this. See details in Jack's comment: https://bugs.kde.org/show_bug.cgi?id=438750 After the total amount is shown for each category, the user can clearly see which category has zero.
Changing resolution status, as there is no bug, and it would be a valid wishilist, but the workaround is easy enough.