Like most good effects this trick is extremely simple. The magician wears a small mirror on the inside of his ring. He merely needs to look at the index of the card in the mirror and name it aloud.
The magician should point out to the audience that the deck and glass are both unprepared and could be borrowed. The deck can be shuffled by any audience member.
A small dental mirror works well, when creating a mirror ring. Instead of a ring, a flesh colored band may be used.
This simple yet confounding trick was revealed to you, courtesy of the Great Mysto!