UINT IsDlgButtonChecked( int nIDButton ) const;
nIDButton
Specifies the integer identifier of the button control.
If the CWnd object is a button control, the IsDlgButtonChecked member function determines whether it has a check mark next to it. If it is a three-state button control, it determines if it is dimmed, checked, or neither.
Nonzero if the given control is checked, and 0 if it is not checked. For three-state buttons, the return value is 2 if the button is dimmed, 1 if the button is checked, and 0 if it is unchecked.
::IsDlgButtonChecked