In order to get to this menu, scroll down to the very bottom of your email inbox. You'll see a small "Details" link on the right hand side.
At the very top of the Activity information screen that pops up in a new window, you'll see a "Sign out all sessions" button. Keep in mind that means any other sessions you may have open on another device will be logged out, and to do a complete sign out of your account you will also need to log out of the current instance that you are in.
Below that, you can see activity as well as location information that indicates where your sessions are logged in from and what device is being used such as a browser, mobile device or a POP3 email connection.
Want to be alerted for suspicious activity on your account? Go ahead and scroll down to the bottom of this screen, and you'll see a link to change your account access alert notifications.
It's probably good to note here that if you log in to many different devices from many different locations that these alert settings may not be for you. But if you are concerned about the potential of having unauthorized access problems in your account (i.e. someone actively trying to hack you) it might be ideal to keep this setting enabled.
You can read more about account activity from this Google resource site. Were you aware that you could access your account activity information from the bottom of your inbox? Is it something that you might find useful?