ronburk - 6:18 pm on Mar 6, 2006 (gmt 0)
You need to be able to examine the mail headers. Most all can be easily faked, except the IP address of the server that passed the message to your mail server (we're assuming your mail server is trustworthy). Examine that IP address and use it to (e.g. www.arin.net) see if it is an address that belongs to Google.