If it is a toby expansion board it will look like https://www.gumstix.com/store/product_info.php?products_id=230.  I would suggest connecting a usb cable to the "console" usb port (which is the top left usb port in that picture) and connecting via serial - 115200 8N1 - and then you can login a shell and find out the IP address or the current state of the system.

From: Lou Morda [mailto:lmorda81@gmail.com]
Sent: Tuesday, 13 December 2011 10:33 AM
To: General mailing list for gumstix users.
Subject: [Gumstix-users] Gumstix Default IP Address Range

Hello Gumstix community,

I've been handed a Gumstix Overo board from a co-worker.  He last used the Gumstix a year ago and has no idea what it's IP address is.  I have it connected to my PC using an expansion board hat only has an ethernet jack, I think this is a Tobi expansion board.  I'd like to use nmap to find the IP address, and I was hoping someone can give me a range of IP addresses to try!  Is there a default range of IP addresses Gumstix's are assigned before leaving the factory?
CAUTION: This message was sent via the Public Internet and its authenticity cannot be guaranteed.

PROPRIETARY: This e-mail contains proprietary information some or all of which may be legally privileged. It is intended for the recipient only. If an addressing or transmission error has misdirected this e-mail, please notify the authority by replying to this e-mail. If you are not the intended recipient you must not use, disclose, distribute, copy, print, or rely on this e-mail.