How do I know, without looking at the properties of every Exchange Online mailbox, which users have setup mail forwarding on their mailbox? That is a very good question. If you Google around you'll find lots of interesting answers, particularly around using LDAP queries to identify those mailboxes - but how do you do that in Exchange Online??? PowerShell is the answer!
First, here is how you connect to your Exchange Online tenant: http://technet.microsoft.com/en-us/library/jj984289(v=exchg.150).aspx.
Second, run this command:
Get-Mailbox -Filter {ForwardingSmtpAddress -ne $null}
That will give you a list of all the users that have enabled (or have a not null) forwarding address configured. Taking this one step further you can grab the user and the destination address by using this command:
Get-Mailbox -Filter {ForwardingSmtpAddress -ne $null} | foreach {$recipient = $_; $forwardingsmtp = (Get-Recipient $_.ForwardingAddress).PrimarySmtpAddress; Write-Host $recipient.Name, $_.ForwardingSmtpAddress }
Ultimately you'd probably want to put this in a script of it's own an pipe the results to a text file for further analysis.
Lastly, don't forget to disconnect your PowerShell session - remember, you can only have 3 open sessions to Exchange Online.