At the moment, there are no simple automated way to do this. Maybe you could verify the visitor's identity manually: enable pre-survey, so the visitor will be forced to enter an email address, and then manually send him some kind of verification question.
However, one will be able to write the appropriate plugin for custom pre-chat survey with the data validation for the upcoming Mibew 2.0. Moreover, one could start developing such plugin right now, since we've made available
the boilerplate plugin and are ready to answer related questions.