In addition, if you log into your Gmail account, you can scroll down to the bottom of your inbox, and at the bottom right you will see "Last account activity X minutes ago. Details." Click on details and you can see the IP addresses that have accessed your email.
If you have any doubts whether your Google account is secure, you should change your password immediately. Also, if you don't use a lock screen on your Android phone, your account will NEVER be secure. Anyone can look at your messages, emails, photos, etc. just by picking up your phone and going through it.