There is small icon by asset:
That little circle before filename. That is Editor trying to be clever and figure out if asset is being used. But due to complexity of where and how it can be used, for example referenced in scripts, it is practically impossible to guarantee that asset is not used.
So treat this circle as "possibly unused"
You can then search for
*. in assets panel, which means search using regexp (starts with
. in regexp means "any character", and select type of what you are looking for. You get all assets of a project, then you can select whatever you believe can be deleted.
As mentioned - it is scary to give button to users "delete stuff", where there is no guarantee to be specific about what to delete.
Perhaps if there would be a search tool, that would allow to search for "potentially unused" stuff, that would allow you easier to see this content, and decide to delete it.