How do I test if object is "pined" to specific imagepoint ?

I have an square with 4 image point ( corners ), when touch square it turn 90degres

and there is a ball dispenser ( every x second create ball with bullet behavior, when the ball overlap the square if there is no ball at the imagepoint where it's overlapping it gets pinned

So how can I test if there is a ball pinned at specific imagepoint?

https://www.dropbox.com/s/8rcc157f1trrd ... .capx?dl=0

Thank you so much for your help, this gives me a lot to play with and Kudos for the capx title XD