Facebook is rolloing out a feature to help users ;protect and manage their accounts. Under "Account Activity" in the "Account Security" section of the account settings page, Facebook will allow you to view all of the active sessions on your Facebook account, i.e. you can now see a list of places where you're currently logged in, along with some info to help you determine whether each session is legitimate or unauthorized.
You'll also be able to view information about the type of device using your account. The device name, type, and location, as well as when it was last accessed will all come up for your viewing. From there you can easily end the activity if it's unwanted, or if you forgot to log off.
Based on approximate location (determined via IP address), the time Facebook session started, and the browser and OS, you should be able to see if you forgot to log out, or someone else has accessed your account. You can also sign up to be alerted via email or text when someone tries to log in from a device that you haven't registered with Facebook.