How can I know whether there are other sessions open on my account

If you are concerned that someone else may have access to your account, for example because you may have left your account signed in on a public PC at a library, coffee shop, etc., click on Account > Account Settings from your Dashboard page. Scroll down to the Sessions section and click on Manage your sessions. Click the Clear Sessions button below to log out of all sessions besides your own current session. You may also want to reset your password, on your Account Settings page.