Count board damage first
Only count minions that can attack face. If a taunt blocks them, either ignore that damage or count the damage needed to clear the taunt first.
Beginner guide
How to calculate lethal in Hearthstone
Most missed lethal turns are not about genius combos. They are usually a counting problem under time pressure. Use the same order every turn and you will catch more wins.
Add damage from hand
Direct spells, charge/rush-to-face effects, buffs, and cheap pings matter. Do not forget mana cost and spell damage.
Add hero sources
Weapons, hero attack, and hero power are easy to forget when the rope is burning. Count them separately.
Check the route
Enough damage is not always enough. Make sure the order works: buffs before attacks, spell damage before spells, and board space before summons.
Quick habit
Say it out loud: board, hand, hero, blocker, order. Then compare the total with enemy health plus armor.