IGDev
It depends on your game - so here just some thoughts
You could store a dead/alive value and the UID in an array.
Or make some instance variable of your enemies (like dead/alive and xEnemy number) - and store this in an array
Or when you create the enemy you store the UID in the arrays and when destroyed delete it out of the array.
if you provide a capx example I would have a better clue.