Microsoft Forms - Transfer Form Ownership

 To transfer forms for users that are no longer active, just go to:

https://forms.office.com/Pages/delegatepage.aspx?originalowner=jdoe@email.com

And select which forms you'd like to move, and where to move them!

Microsoft Forms - Finding the Owner of a Form

To find the owner of a form, open up the Developer Tools (F12) in your browser, go to the Network Tab -> Fetch/XHR, then look for the one that begins with "ResponsePageStartup", and go to Preview.  Expand  the form data and you can grab the owner id:

Once you have the ID, you can look it up in Azure AD to find the user.