If destroyed? how to check if destroyed?

i have 12 separate gameobjects that the player must destroy in order to advance to the next level.
how would i go about checking if all 12 of these objects are disabled/destroyed?
I can get it to play the animations and sounds properly but how would i set an if statement to see if they are destroyed?

You could try to create a script with an array of the game objects and everytime one is destroyed (if GambObject==Null) increment a variable in the script when the variable becomes a certain number do something etc