I find out which Shared Mailboxes a person has access to?

This is probably as much an Exchange question as an Active Roles one, but is there any way to find out from Active Roles which Shared Mailboxes a user has access to?  We're sometimes asked to make a new user the same as an existing one and they don't always include all the mailboxes they want, or when a person moves department we'd like to find an easy way to identify this to remove them.  It might also be useful to use as a form of auditing if we could email each user with a list of the mailboxes they have access to in order to clean up any they no longer require.  We don't automap mailboxes so I believe there will be users with access to mailboxes they are not aware of and no longer need.

We're using Azure, Exchange Online and Office 365 so there's no Mailbox Sharing tab available to interrogate.  Our accounts are created and maintained by the First Line staff and they have no access to Exchange Online or Azure to run Powershell queries against it so at the moment they need to ask the Exchange administrators.