System Delphi

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;