I imagine it would require having a script on the actor, then checking the actor's health at relatively short intervals, and then comparing the value to the actor's health in an OnHit event.
Sounds as if it you would have to make a decision between accuracy and processing power there. The faster you check at short intervals, the more intensive the script would be, yet slower intervals would leave much more room for error.
Then there is the complex task of dynamically applying this script to actors that the player is near.