Count lethal and evaluate a damage race
Count the damage you can really deal. Check that you can survive long enough to finish the game.
Count a winning turn
Lethal means enough damage to win. Add your ready minion attacks, damage from hand, weapon attacks, and Hero Power.
Then check the limits. Do you have enough Mana? Must you clear a Taunt? Can every attack hit the target you need? Did you include Armor? Does the order change the damage?
Keep sure damage separate from lucky draws or random effects. A minion you just played may not be ready to attack.
What is a damage race?
A race is a game where both players try to finish the other first. Your “clock” is how many turns you seem to need to win.
That count can change. Healing, removal, Taunt, and damage from hand can all change the race.
Suppose the other hero has 12 total Health and Armor. You can deal seven now, leaving five. That plan is good only if you can survive and find the last five damage. If the enemy can win next turn, you may need to trade instead.
Keep a way to finish
Board damage can vanish when minions die. Damage in hand may be safer to save. But you might need that spell to remove a Taunt now.
Count visible enemy damage and a reasonable amount of extra damage. Do not fear every possible card. Choose a plan that protects you and still gives you a way to win.