Quick an simple power shell line to list all shared mailboxes in your Exchange 2013 environment.

 

What’s a Shared Mailbox

A shared mailbox is a mailbox that multiple users can use to read and send email messages. Shared mailboxes can also be used to provide a common calendar, allowing multiple users to schedule and view vacation time or work shifts. ref: http://technet.microsoft.com/en-us/library/jj150498%28v=exchg.150%29.aspx