To know when the deck is favorable for you in blackjack, you use a card counting system, such as Hi-Lo. As the cards are dealt, you assign values to them (positive for low cards, negative for high cards, and neutral for 7-9). When the running count is positive, it indicates that more high cards (10s and Aces) remain in the deck, which is advantageous for the player. This means you should increase your bet and play more aggressively. Conversely, if the count is negative or neutral, it suggests the deck is more favorable to the dealer, and you should bet smaller or play more conservatively. In multi-deck games, adjusting the running count for the number of decks remaining gives you a more accurate picture of the deck’s favorability.