There are two areas on a document that display whether it is shared and with whom it is shared.
1. If the document has been shared via shared folders, it will be visible via breadcrumbs at the top of the document. The breadcrumbs will also indicate that the document is private if that is the case.
2.You can click the share button in the top of the document to display the folders and individual people that have access to the document
Whenever a document is shared with a person/folder or a person is removed from a document, there is also a record of it in the thread.
You can refer to the thread to see the history of who has had access to the document.
Shared folders have an icon on them to indicate that they are shared while the private folder will have a lock icon.