Concentration checks are made when a caster is concentrating on a spell and gets hit with an attack:
1. Determine if half of the damage dealt by the attack is higher than 10, rounded down. Attacks that deal 22 or more damage in a single strike will qualify here.
2. Set the DC of the check at 10, or half of the damage dealt by the attack. Whichever is higher.
3. Make a Constitution saving throw: D20 + CON ability modifier + prof. Bonus
4. If the saving throw meets or succeeds against the DC, the caster retains concentration on their spell
This check must be made every time a caster is hit with an attack. For example, if a caster is hit with a 1st level magic missile they must succeed in three DC 10 concentration checks.