Steps for Checking Your Gmail Account is Hacked or Not
1. Log in into your Gmail using your username and password.
2. Click on "Details" link on bottom-right of the page."Details" link appears below "Last account activity" on bottom-right of the page.
3. In Recent Activity Window you see when you logged in from Which IP. Recent activity also show locations of your logged in Computer.
4. Click on "Sign out all other sessions" if you are unsure that you have not logged out from your account. By clicking this option Gmail will sign out all other sessions from which you have logged in.
This is the basic method to know your gmail is hacked or not.