Using DeliverPoint you can produce a SharePoint Permission Report detailing exactly all users who have permissions to the given site, list/library, folder or item. Every user with permissions to the objects will be included in the report and can be sorted upon or filtered. You will see exactly how the user received permissions even if it is via an Active Directory Group. The reports can be exported to Excel or produced via a schedule which can be configured for daily, weekly, or monthly intervals. Learn more about the SharePoint Permission Report here.