Well, here's a quick sample of what you can do with PowerShell:
Get-Mailbox -RecipientTypeDetails SharedMailbox |select PrimarySmtpAddress,@{n="FullAccess";e={ (Get-MailboxPermission $_.PrimarySmtpAddress | ? {($_.User -ne "NT AUTHORITY\SELF") -and ($_.IsInherited -ne $true) -and ($_.AccessRights -match "FullAccess") -and -not ($_.User -like "S-1-5*")}).User -join "," }} |? {!$_.FullAccess}
It's a one-liner, so a bit ugly, but should get the job done. Do note that it will take a long time to run if you have a large number of mailboxes. Also it doesn't cover any folder-level permissions, if you want these included best go with a full-blown script. I have few samples posted on the TechNet Gallery, you can easily adapt them.
