...trap the Main forms WM_MOVE message?

type
  TForm1 = class(TForm)
  private
    procedure WMMove(var Msg: TWMMove); message WM_MOVE;
    { Private-Deklarationen}
  public
    { Public-Deklarationen}
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.WMMove(var Msg: TWMMove);
begin
  inherited;
  ShowMessage('Form has moved!');
end;

end.