获取各种编码的识别符

下面是常用编码的识别符, 在 Delphi(2009) 中如何获取呢?
Unicode: FF FE; BigEndianUnicode: FE FF; UTF8: EF BB BF

---

var

  bs: TBytes;

  b: Byte;

  str: string;

begin

  {只有 Unicode、BigEndianUnicode、UTF8 编码有识别符}

  bs := TEncoding.Unicode.GetPreamble;

  str := '';

  for b in bs do str := Format('%s %x', [str, b]);

  ShowMessage(str); {FF FE}



  bs := TEncoding.BigEndianUnicode.GetPreamble;

  str := '';

  for b in bs do str := Format('%s %x', [str, b]);

  ShowMessage(str); {FE FF}



  bs := TEncoding.UTF8.GetPreamble;

  str := '';

  for b in bs do str := Format('%s %x', [str, b]);

  ShowMessage(str); {EF BB BF}



  {ASCII、UTF7 和 Default(默认编码) 没有识别符}

  bs := TEncoding.ASCII.GetPreamble;

  str := '';

  for b in bs do str := Format('%s %x', [str, b]);

  ShowMessage(str); {无}



  bs := TEncoding.UTF7.GetPreamble;

  str := '';

  for b in bs do str := Format('%s %x', [str, b]);

  ShowMessage(str); {无}



  bs := TEncoding.Default.GetPreamble;

  str := '';

  for b in bs do str := Format('%s %x', [str, b]);

  ShowMessage(str); {无} 

end;