By the naughty smile on his face. His reaction gives him away.
I’m mentioning this today to you magicians in the Philippines, because not everything that happens on stage needs to be seen first-hand by the entire audience. It is not practical to do so. It is time-consuming. Above all, it is unnecessary.
If you bring one person on stage to examine a box, for example, you can make the entire audience believe that the box is empty by how well that one person reacts to what you show him.
Don’t blow your chance to amaze the audience by mishandling a volunteer on the stage. If you want to prove the box is ungimmicked, make him declare, in clear and unequivocal manner, that the box is empty. He can do this by words, by facial expressions, or by the glow of his eyes.
Don’t allow him to be unresponsive. If he is as numb as a cadaver and slow to react, egg him with words like, “Does the box have a whole big enough for a person to pass through?” If he mumbles a reply, command him, “Say it loud!”
Next time you bring one person on stage to inspect a prop, make sure he reacts appropriately. If he does not play his role well, remember the picture above. Be the girl who is reflecting an interesting image. Make the volunteer react. It is your duty to ensure that he gives out the appropriate reaction, otherwise you will lose the audience in the shuffle.