...detect changes in display?

type
  {...}
  private
    procedure WMDisplayChange(var msg: TMessage); message WM_DISPLAYCHANGE;
  public
  {...}
  end;
end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.WMDisplayChange(var msg: TMessage);
begin
  ShowMessage('Display settings changed!');
  inherited;
end;