Forms History lets you see if changes have been made to a Google Form, create back-up versions, and access previous copies.

Use Forms History if:
 1) You would like to see if someone has modified a Google Form.
 2) You want to create and access back-up copies of Google Forms.

Important: Only copies of the form made with Forms History are available in the version history.

GETTING STARTED
(1) Install the Forms History add-on.
(2) At the top of your Google Form, click Add-ons (puzzle icon) >  Forms History > Open.
(3) If this is the first time you are using Forms History, you will be prompted to provide the necessary permissions.
(4) The Forms History panel will show the form revision history 

For More Information: 
https://hawksey.info/forms-history/help/

Acknowledgements - Thanks to Alice Keeler (alicekeeler.com) for Forms History feedback, ideas, artwork and inspiration.