Contrary to popular belief, card counting is not memorising what cards have been dealt so that you can work out what cards are left in the deck(s). To begin with, unless you're Dustin Hoffman in Rain Man, you can't keep track of what's been dealt from six decks of cards. On top of this, only four or five of the decks are actually dealt before a reshuffle.

In short there is no certainty, BUT there are ways to estimate the ratio of cards left in the deck.

If there are a lot of lower cards, say 6s and below in a deck, then your play should start to change.