You know you are finished when you file your return or print it out to send it by mail. Both options are found on the Filing Hub page (under File at the bottom of the left side menu).
If you send your return via NETFILE, you'll get a confirmation number when it has gone through successfully. This FAQ tells you how to submit your tax return to CRA using the NETFILE service in TurboTax Online: https://turbotax.community.intuit.ca/community/federal-taxes-netfile/help/in-the-online-edition-of-turbotax-how-do-i-submit-my-tax-return-to-cra-using-the-netfile-service/00/873715