If you signed in to Google on a public wifi (RISKY!) but forgot to sign out before leaving or if you've gotten notification that there's been a login and you don't know what it is, there's a way to see all open sessions of your email.
Click the tiny Details link at the bottom right hand side of your inbox.
You can look at the details of each open session. If there's one from a location or device that you don't recognize, click on Sign out all other web sessions.