if Cube is found then do if statement

how do i make a script that looks if a Cube had been instantiated, then we will do an action inside the if statement. How would i do that, im trying to make a screen that says loading world, and when a cube is found then remove the screen.

For this to work you will have to call this function from

function Awake(){
//put the code here
//do all you checking here only if it needs to be done before showing the screen
//this loads before the first frame updat
}