Riddle

The third applicant was placed behind the other two, so that he could see the names on their handkerchiefs bur neither they nor he could see the name on the handkerchief that was placed on his back. Bentley, of course, could not see any handkerchief. "the first one of you who can deductively determine the name on his own back will get the part," the director announced. After a few minutes of silence, Bentley correctly asserted that he was Sherlock Holmes. How did he know?