How to know if the user does not finish or completes the form?

Trying to figure out if there are any local PC files that are generated that can be recovered if a user does not finish a form or claims to have done so. 

You can set up "save and continue forms later" if you want to confirm whether your users completed the form or not.

Here is the guide for that: How-to-Save-Forms-and-Continue-Later

If the user does not submit a form, the submission automatically goes to an incomplete submission and it will be logged.