How do you know, in your in-person whiteboard interview, that the candidate isn't wearing an earpiece and a microphone, and someone else is whispering a solutio

How do you know, in your in-person whiteboard interview, that the candidate isn't wearing an earpiece and a microphone, and someone else is whispering a solution, and convincing explanatory patter, into their ear? Do you make them whiteboard in a special room that's set up as a Faraday cage?