Only one Google account can "own" any Google doc/sheet/form/slides, etc. So, it is a best practice to encourage co-workers to share their files with one another. If the form has been shared with you, you can see who owns the form. The owner can also transfer ownership to anyone with whom the file is shared. If you need to modify a form to which no owner can be found, you will have to make a new form.
Supervisors should work with exiting employees to transfer ownership of all of their Google Drive contents.