If you log-in to your Gmail account on computers that aren't yours, you're probably very responsible about logging out afterward so that no one can steal your e-mail account. But what about that one time you were drinking too much at the library (again) and you can't remember if you logged out of the public terminal?
to find out if you're logged in anywhere else, and what to do if you are. Scroll down to the bottom of your screen and click details.
You'll get a pop-up window listing all the other IP addresses that have logged into your account. If they're all the same, they're probably just the computer you're on, and you're OK. However, if you see a different one, you may have a problem.
To be sure, click "Sign out all other sessions" and all but the account you're currently using will be kicked out. If you're worried at all that someone may have figured out how to access your account, be sure to change the password right away.