Weblink respondents are anonymous, unless you use the 'c' query string parameter to identify them.
Email respondents can be identified by their email address.
Both identifiers can be retrieved using get_respondent_list - you can then use the respondent id associated with that response to retrieve the specific response using get_responses.
Note that identifying people using the 'c' parameter appears to be deprecated now - it is no longer listed on the SurveyMonkey help center. It still technically works, but is not recommended for new development. The supported way to do this is via custom varilables which requires a platinum account. You then have unlimited variables to track users, and these are returned in the get_responses response.