Forms History lets you see if changes have been made to Google Forms™, 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 Form History are available in the version history.

GETTING STARTED
(1) Install the Form History add-on.
(2) At the top of your Google Form™, click Add-ons (puzzle icon) >  Form History > Open.
(3) If this is the first time you are using Form History, you will be prompted to provide the necessary permissions.
(4) The Form 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 Form History feedback, ideas, artwork and inspiration.