Extracting Text from a Dialog Window

How can I extract text from an existing Dialog Window? Its a 3rd party dialog window so I don't have the source code of it. I have to extract text from its Lables and Text box. I think for extracting text from TextBox we can use WM_COPYDATA message (Not sure) But how can I extract text from labels of a dialog Window?

If I am not wrong WM_COPYDATA is used to pass data to another application. What I am trying to achieve is reverse of WM_COPYDATE where I have to extract text data from Labels, TextBoxs, Radiobuttons, Checkboxes and other dialogBox Controls.

Getting the Text of a Window in Another Application