How does the manager check the departments leave calendar

There are a few ways depending on when this is being done.

Either go to My details > Groups > Select the group name that you want to see the leave for and select Leave (dropdown) > List of or Calendar.

Or as the Manager is deciding to approve the leave.  In this case click the Calendar icon in the 'Leave Request To Be Approved'.  This will open a new window with the leave calendar for the group.