how are anonymous participants identified
If you simply create a public survey, participation will be anonymous and you will not be able to identify the respondents - because that is the idea of "aonymous".
If you would like to offer the respondents an option to ask for deletion of their data set, you can show them their CASE number for example:
<p>Your person code is %caseNumber%.</p>
Or you could also use secure code, see Display individual codes or voucher codes