注意,此方法并不完美,如果您有更严格的编码判断需求,推荐使用 UTF-unknown 这个类库!
' 获取文件编码,原C#编码,本人改写为VB.NET代码
' 原文:http://www.devbook.cn/newsdetail.aspx?news_pkid=N100008
' 本人主页:https://blog.clso.fun

Imports System.Text, System.IO

''' <summary>用于取得一个文本文件的编码方式(Encoding)</summary>
Public Class TxtFileEncoding

    ''' <summary>用于取得一个文本文件的编码方式(Encoding)</summary>
    Shared Function GetEncoding(ByVal filePath As String) As Encoding
        Return GetEncoding(File.OpenRead(filePath), Encoding.Default)
    End Function

    ''' <summary>用于取得一个文本文件的编码方式(Encoding)</summary>
    Shared Function GetEncoding(ByVal stream As FileStream, ByVal defaultEncoding As Encoding) As Encoding
        Dim targetEncoding As Encoding = defaultEncoding
        If stream IsNot Nothing AndAlso stream.Length >= 2 Then
            ' 保存文件流的前4个字节
            Dim b1 As Byte = 0, b2 As Byte = 0, b3 As Byte = 0, b4 As Byte = 0
            ' 保存当前Seek位置
            Dim origPos As Long = stream.Seek(0, SeekOrigin.Begin)
            stream.Seek(0, SeekOrigin.Begin)
            Dim nByte As Integer = stream.ReadByte
            b1 = nByte
            b2 = stream.ReadByte
            If stream.Length >= 3 Then b3 = stream.ReadByte
            If stream.Length >= 4 Then b4 = stream.ReadByte
            ' 根据文件流的前4个字节判断Encoding
            ' Unicode {0xFF, 0xFE};
            ' BE-Unicode {0xFE, 0xFF};
            ' UTF8 = {0xEF, 0xBB, 0xBF};
            If b1 = &HFE And b2 = &HFE Then targetEncoding = Encoding.BigEndianUnicode
            If b1 = &HEF And b2 = &HEF Then targetEncoding = Encoding.UTF8
            ' 恢复Seek位置       
            stream.Seek(origPos, SeekOrigin.Begin)

        End If
        Return targetEncoding
    End Function

End Class