If you require the identity of your respondents to be 100% reliably confirmed on your forms, you need to enable "Login" protection on your forms.
What this does is present your form visitors with a login screen before they can get into your form, and this way you are 100% certain of the e-mail identity of the person who submitted the form. It will also mean that the submitters verified email address will show up in your reports as the form submitter, rather than "Anonymous" being listed as the form submitter.
Steps for enabling "Login Protection" on a form are here.
One other benefit of enabling login protection for a form is that your users will have access to their own User Portal, to be able to securely view and print their past submissions.
Alternatively, in order to identify your form respondents you could also pass variables in the URL querystring to pre-populate fields which you've set as "hidden" on the form and pass identifying data this way - but please be aware that technical users are able to find out and bypass this so this should only be used in scenarios where it's OK for the identifying information to be sometimes inaccurate. If you need 100% certainty though, the way to do this is by enabling "Login Protection" on your forms.