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;