The Mailboxes report is a great place to start to get a handle on the mailboxes within your organization and some of their base information, incuding location (In Cloud or On-Premises)
The report is in table form. It is a complete listing of all the mailboxes, including if they are on-premises or in the cloud. In order for on-premises information to be gathered you have to deploy the on-premises agent.
As with all of the reports, this information can be customized, filtered and scheduled.