You can tell that a channel is private because it will have a lock padlock icon next to its name. It will also have the additional options to "Add members" and "Leave the channel" in its menu. Channels can have permissions and some other settings that are distinct from the parent team and from other channels in the team. To manage a channel's settings select the drop down menu from the ellipses next to the channel name. and chose the option you want. | 
