You may detect it by one of the following events:
- foreign trades/payments in your which weren't performed by you;
- incoming notification emails which weren't requested by you;
- foreign sessions and/or unknown activity in the security log.
If you see some of these, follow steps below immediately:
- Change your password and terminate all sessions.
- 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 a situation again.