You can look at the message header itself and that should help, but even that's not always helpful.
If you are using Outlook you can right-click on the message and select "option" of "properties" depending on the version. It will look complicated, but if you look closely at the "From" fields, you can figure it out.
Just keep in mind, if it is SPAM, the "From" fields will probably be inaccurate, but they should show someone's address or server other then yours. The spammers usually bounce emails off of open mail servers.
[edit: Sorry I just noticed you're using gmail. The principles are the same, but I'm not sure how you can examine the header info in gmail]