The following code works fine for me, using the StreamReader class:

  using (var reader = new StreamReader(fileName, defaultEncodingIfNoBom, true))
  {
      reader.Peek(); // you need this!
      var encoding = reader.CurrentEncoding;
  }

The trick is to use the Peek call, otherwise, .NET has not done anything (and it hasn't read the preamble, the BOM). Of course, if you use any other ReadXXX call before checking the encoding, it works too.

If the file has no BOM, then the defaultEncodingIfNoBom encoding will be used. There is also a StreamReader constructor overload without this argument (in this case, the encoding will by default be set to UTF8 before any read), but I recommend to define what you consider the default encoding in your context.

I have tested this successfully with files with BOM for UTF8, UTF16/Unicode (LE & BE) and UTF32 (LE & BE). It does not work for UTF7.