Have you used the ‘last account activity’ feature of Gmail yet?
If you are suspicious that your account has been hacked you can make use of the 'Last account activity’ feature to check for irregular account behavior
This feature is a simple one-click process that gives a fair idea about whether a Gmail account has been hacked or not. With the help of this feature, Google shares with a user the date and time at which the account was accessed. Additionally, the IP address of the device that accessed
the account, the associated location of the device and the nature of the device (like a browser, mobile, third-party application etc.) are also mentioned. The user is also informed if the account is currently open at another location or if the account has not been signed out.
See the bottom Right Cornor and Click Details for usage Details.