Since you can open the device, it isn't passcode locked, so there are a few things you can try to contact the previous owner.
The email should be listed in Settings under iCloud, iTunes & App Store or Mail, Contacts, Calendars
Other Apps you can check to find contact info:
1. The mail App: Open any Sent mail and you can tap and hold on the "From" to see the email address.
2. Check the Contacts App: The partial email address may be complete in contacts or linked to another email you can use. With any luck the card may include a name and address and phone.
3. Facebook; Facebook has the facebook email in the "About" section. this may also help if there are photos of the owner, etc.
Good luck!