//Ansi文件开头直接是文件内容,字符编码;Unicode文件开头两个字节是FF FE
FileStream fs = new FileStream("C:\\tempUnicode.txt",FileMode.Open,FileAccess.Read);
byte[] buffer = new byte[2];
fs.Read(buffer,0,2);
fs.Close();
string h = BitConverter.ToString(buffer);
if (h=="FF-FE")
{
//Unicode
}