You may detect it by one of the following events:
foreign trades/payments in your Reports which were not performed by you.
incoming notification emails that were not requested by you.
foreign sessions and/or unknown activity in the Security log.
If you see any of the above, please follow the steps below immediately:
Change your password and terminate all sessions here.
Enable 2-factor authentication, if you haven't already.
Change the password to your email account.
Check your computer for malware. Make sure that you are not using a public/unprotected WiFi connection.