In Layout mode identify any object with assigned local style format.
How To Implement
There are multiple ways to implement this features (highlight objects in layout, …), but I think, the best would be in the Layout Objects window. Perhaps add a new entry in the the filter popup "Local CSS" in such a way that you can select an object type (e.g. "Fields") and "Local CSS". Result: the filter will show only layout objects of the type Field with any local CSS.
This will help the developer to work more efficient with styles and get rid off local CSS.