This is a really simple PowerShell script which will list all mailboxes on Office 365 with Forwards configured.
$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session -DisableNameChecking
Get-Mailbox | Where {($_.ForwardingSMTPAddress -ne $null) -or ($_.ForwardingAddress -ne $null)} | Select Name, ForwardingSMTPAddress, ForwardingAddress, DeliverToMailboxAndForward