Look at the discard tray when the dealer loads it up just before shuffling. Note the top of the stack of cards in relation to the top of the shoe. Some stacks come to the top of the tray and some are an inch or so down from the top depending on the tray. If you are looking at 6 decks stacked, then half way down is 3 decks and half of that is 1 1/2 decks. It is then easy to estimate the cards in the discard tray. You don't have to move your head to see the discard tray, just a glance out of the corner of your eye does it just fine. No one can tell you are looking at it.