function UrlCheck(sUrl: String; bAllowRedirects: Boolean = false): Integer;
begin
  with TidHTTP.Create(nil) do
  try
    try
      HandleRedirects := bAllowRedirects;
      Head(sUrl);
      Result := Response.ResponseCode;
    finally
      Free;
    end;
  except
      Result := GetLastError;
  end;
end;

begin
  if UrlCheck('http://www.delphipraxis.net', true) = 200
    then ShowMessage('!')
    else ShowMessage('?');
end;