Checking for Object Presence

How can I check for the presence of something on the stage? I want to disable mouse clicks on the currentTarget while an image is loading, and I’d like to use the presence of a progress bar as the determinant to proceed.

My idea is:

 
If (progressBar is somewhere on the stage)
{ 
currentTarget.mouseChildren = false;
currentTarget.mouseEnabled = false;
}

Does that make sense?