How do I remove dangling asset references?

UATHelper: Packaging (Android (Multi:ASTC,PVRTC,DXT,ATC,ETC2,ETC1)): Cook: LogPackageName: Error: DoesPackageExist: DoesPackageExist FAILED ‘/Test/Test1’ is not a standard unreal filename or a long path name. Reason: Path does not start with a valid root.

The error message is correct the path does not exist - it (a plugin) has been deleted. However, there seem to be dangling references in some other assets. How might i identify and remove those?