To make sure you aren't sharing inadvertently, you need to see at a glance what files or Tasks can be seen by people outside of your Organization.
Look for this icon on any Task or File: . It means that the item has been shared with at least one person outside of your organization. Hover over the item to see who can view the item.
To sort a list of Files or Tasks to only show the items that have been shared, select the Shared Tasks icon at the top of the screen.
To learn how to change permissions on an item, see this article.