In the ‘raw view’ of any mail message scanned by Avast Mail Shield should be X-headers. Below are headers from a message that I’d sent and then received back a couple of weeks ago.

X-Antivirus: avast! (VPS 131213-0, 12/13/2013), Outbound message
X-Antivirus-Status: Clean

X-Antivirus: avast! (VPS 131213-0, 12/13/2013), Inbound message
X-Antivirus-Status: Clean