To keep tabs on your account activity, make sure you have “Email me about account activity” turned on in account settings so you’ll get an email when someone logs into your account, your password is changed, or a new app is authorized.
If you want a more hands-on approach, scroll to “Active Sessions” a little further down on the account settings screen. It shows a list of browsers and locations that have accessed your account over the past 30 days. If you see anything you don’t recognize, you can end that session immediately by clicking the gray X next to it.
Note that Active Sessions shows mobile activity when it’s coming from a mobile web browser, but not when you’re using the app. You can always view and manage your authorized mobile apps on the apps settings page.