If you navigate to your PROFILE page you will see a list of current user sessions. This list shows each of the current logged in sessions for your account, including your current session which is highlighted in green. Each session has created date, accessed date, IP address and the browser being used to help you identify if the session is valid or not.
If you find a user session which you don't recognise, you can revoke it by clicking the red REVOKE button. This will immediately log the session out so it cannot be accessed anymore.