Well, duhhhhhhhhhhhhhhhhhhhhhh! Andrew Nusca just wrote about the very obvious for the umpteenth time.
Ever notice that a lot of poker players playin’ Texas Hold’Em wear dark sunglasses? There a lot of ways to ascertain what your opponent is doing: a little reverse psychology, unnerving your opponent, detecting microcosms in the throat, etc. And sometimes your opponent can’t play for shit to save his life savings – that’s a dead giveaway,too.