You can run a search for entries that do not have a tag. A crude work-around for manually-selected items would be to assign them all to a new tag & run a search for entries that do not have that tag.
I just made a script for this purpose using the Actions and Tags plugin. First install the plugin if you haven't, then install my script. Finally use the script just like any other scripts for the Actions and Tags plugin.
Upgrade Storage
First install the plugin if you haven't, then install my script. Finally use the script just like any other scripts for the Actions and Tags plugin.
Links:
Actions and Tags plugin: https://github.com/windingwind/zotero-actions-tags
The script: https://github.com/windingwind/zotero-actions-tags/discussions/508