how to get the value of a control if it 's handle is zero?

Question

I've known that a control gets its handle when it can be seen the first time.
My problem is that in my program , I want to get the value of a  control in anther forms(not created by me).When I use the API "ChildWindowFormPointEx" , it returns zero--it hasn't been seen ever--so I can't get it .
Who can help me? Thanks in advance!