It will vary depeding on the mail client you use. However, based on using Outlook;
right click on the message and select options (message options if your using outlook 2007). At the bottom, under headers, is where it shows all the specific routing information for this message. IF you need to forward this message to your mail admins, don't forward the message. Create a new message and attach it. That way the mail headers will be intact.
It could be very likely that it is indeed coming from gmail. the only things you can do there is to report it to gmail as an abuse of thier mail system. Then let your admins know, so they can add it to the spam filter list (that I hope your company uses).