You may detect it by one of the following events:
If you see some of these, follow steps below immediately:
Change your password and terminate all sessions here.
Enable 2-step authentication, if you haven't already.
Change the password to your email account.
Check your computer for malware. Make sure that you're not using public/unprotected WiFi connection.
By doing this you will take away the access to your account from possible intruders. Make sure that you won't face such situation again. Here is a detailed guidance on how to stay safe and secure.