How to check if a movieclip shown on stage?

Hi guys!

I have a movieclip AAA(W:550xH:1200) placed on the stage (W:550xH:400). Inside of the movieclip, there are many other moveclips.
I like to implement this result:
When I scroll(Y-axis) the movieclip AAA to a position, the moveclips inside of AAA will fade in, and the other movieclips out side of the stage will fade out.
Could any one give me some hints?
THANKS!!