If a layer is in use by a user on SDE and you need to do something with it like Administrate tools, you can go to the Locks and see who is using it. But if you have servers using a layer, which is usually the case, it can be time-consuming tracking down what service is using a layer.
However, what is far more difficult is figuring out what service is using a layer, if you need to do an action where no user can be connected. Sure it will tell you the Server machine. Not useful. We have hundreds of services. While I am creating a tracking spreadsheet many of the services were created before my time but are still running. Tracking down what service is using what layer manually is time consuming.
Create a tracking tool. Simply show a table of what services (and on what servers) are using layers in SDE. Or fgdb.
What webmaps are using a service?
Solution: create a tracking tool on ArcGIS Online to show what webmaps are using a service.