......
function GetTitleBarInfo(hwnd: HWND; var pti: TTitleBarInfo): BOOL; external user32 name 'GetTitleBarInfo'; //Для верности ...
......

procedure TForm1.Button1Click(Sender: TObject);
var pti: TTitleBarInfo;
begin
GetTitleBarInfo(handle,pti);
end;
... 

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

  function GetTitleBarInfo(hwnd: HWND; var pti: TTitleBarInfo): BOOL; external user32 name 'GetTitleBarInfo';
var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.Button1Click(Sender: TObject);
var   pti: TTitleBarInfo;
begin
  pti.cbSize := sizeof(TTitleBarInfo);
  GetTitleBarInfo(handle,pti);
  pti.cbSize :=  0 ;
end;

end.