Basically, your partner is not able to view your desktop without you knowing because by default, TeamViewer will notify you if someone is connecting. However, if you're not prudent and the partner knows your ID and password, it's possible. Also, if you disabled somehow the notification option, you will not know when the partner is connecting. To make sure that this option is not disabled, go to Extras > Options > Computers & Contacts and check the box next to Notify me when partner sign in:
For further information, I suggest you contact the support team.