There are plenty of ways to prove that, including stating a public key during the interview, interviewer signs it with their key, vice versa is done with the interviewer's key, and then the same is demonstrated in person with the same keys.
Hell, how to do that can even be an interview question.
That's a terrible answer to the hypothetical interview question because your solution only works if the person that took the interview is not colluding with the person that wants the job. The interviewee could just email their private key to the job seeker.
The key signing only protects against some very uncommon threats
Hell, how to do that can even be an interview question.