In the product documentation, there is the Live Compare -> Quick Compare where you are able to connect to a site or sub-site to view the drill-down of lists / folders / files for any missing. It will be highlighted as per below legend.
The Live Compare results use color to highlight the differences between items in two SharePoint locations. The color meanings are:
Please refer to the user guide for reference below on how to check differences between Source SPO Document Library and Target SPO Document Library.