Many of the popular websites and services have been or at some point or will be targetted and exploited. As a result, it is most likely that at least one of your online accounts has been pwned at some point. This doesn’t mean your account has been cracked but it could mean that some of your private information has been leaked (e.g. email, password, personal info like date of birth, etc…). Recently Gmail has been exploited by attachement phishing.
There’s no guaranteed way to check if your account has been hacked but one way is to check if you’ve been pwned. Just enter your username or email to check if you’ve been pwned.