Any Gmail account getting hacked or chances of any unauthorized intruder can be gauged in the following ways:-
a) Firstly by checking whether any spams have been sent from the account to any unintended recipients
b) Secondly by checking the account feeds which contains information on the location, time and the IP address from which the account was being accessed
The Gmail Technical Support team have instated automated system which monitors the log in activities of all user accounts and in case of any suspected log in activities, the owner of the account is immediately informed via email. However, in case of any such occurrence, the Gmail Customer Support team should be immediately contacted and their guidance should be seeked.