int pascal CheckDialogItem(HDLG hDlg, int id, int state)

  This function checks or unchecks a button control contained within a
dialog box. This function should be used to control checkboxes, and
CheckRadioButton() should be used to control radio buttons.

Parameters
  hDlg is the handle of the dialog box which contains the specified control.
  id is the identifier of the control that we want to query. (The id
   is not the same thing as the handle.)
  state is TRUE if the control should be checked, FALSE if the control
   should be cleared.

Returns
  TRUE if the operation was successful, FALSE if not.

Example

   CheckDialogItem(hDlg, ID_YELLOW, TRUE);