Delivered-To portion of the email header. This will show the actual address the email was sent to.
It's usually the first line before the rest of the information:
Received: by XX.XXX.X.XXX with SMTP id XXX
Thu, 13 Aug 2009 05:25:18 -0700 (PDT)
To see the full header information, click on the down arrow next to the Reply option on the email and click Show original. This is usually at the top right of the email itself.
Note that some themes and skins may move the menus around.