As the postmaster, you have access to view all messages currently held in quarantine for your domain. When you login as a postmaster, you will be taken automatically to the domain center dashboard.
From there you can access the quarantined email messages for your domain. You can monitor all messages in the quarantine. By clicking on a message you can find the summary information about the message including the sender, recipient, subject, and when it was received. You will also find a button in the top right corner called preview message which will bring up the raw message content. All formatting and html codes are removed for the preview so it is safe to view. This preview will allow you to further determine if the message is actually spam or not.