Run the following command in the EMC to list all mailboxes who have delegates on them.
Get-Mailbox -resultsize unlimited | Where {$_.GrantSendOnBehalfTo -ne $null} | select Name, @{Name='GrantSendOnBehalfTo';Expression={[string]::join(";", ($_.GrantSendOnBehalfTo))}} | Export-CSV C:\SendOnBehalfTo.csv -noTypeInformation