there is no Standard Option or Button or view in PIM which can solve it.
The only thing which already exists is a standard Service which is called when a Image is deleted in MAM. The Service will ask if the Image is still in use and if so he will not delete it. (Administration -> Client -> Customer -> Medias Settings -> PIM URI)
It also belongs to the different use cases, for example some are using Versions some are using the MAM side also to store image which are not related to an Article in PIM.
At the moment it is only possible by SQL so there are tow tables, one in PIM one in MAM, which can be cross checked.
Maybe you can write a Custom Store Procedure which check that tables and writes back a value into a propertie field.
Another option can be to set a properties field in MAM when ever a JMS is send to PIM to get linked to check.
The below SQL returns all Media Manager (MM) images that are not assigned in Product Information Management (PIM). Thought it could be helpful for you, not sure on how you could utilize it on your dashboard.
Select * from opasdb.dbo.F_IMGKOMP where PKOM_PNR not in (select Identifier from HPM_MAIN.dbo.MediaAssetDocument)
Thanks for the update, but that's not what I'm looking for.
The web users should be able to get this overview.