With the rampant spree of cyber criminals one can never be too safe. Keep an eye on your Gmail account activity.
Open your Gmail account
Scroll to the bottom of the page and click Details
A new window will open with all your recent activity. You can also view the locations where your account is logged in to. The list also shows the location of any authorised apps.
To sign out of any open sessions, click the Sign out of all other web sessions button.