The magician puts on a blindfold of solid steel. But even with the blindfold on, the magician can still easily tell what objects spectators are holding.
The steel blindfold, invented by Richard Osterlind, looks like it is made of solid steel, but it is not. While the blindfold is steel on the outside, it is clear, transparent glass on the inside, so even though spectators can't see in through the blindfold, the magician can easily see out.
The blindfold makes possible all sorts of psychic demonstrations, including card divinations, mind reading, and blindfolded driving. A particularly entertaining part of any Osterlind corporate show is when Osterlind, "blinded" by the blindfold, is interrupted during a mind reading demonstration by a team of black-clad ninjas. Osterlind handily defeats them all with just his bare hands without removing the blindfold. Note that, although we do not have definitive information on the subject, we assume that the ninjas are stooges and are generally not really killed.