How to get the encoding type of a file?

Follow the code below:

string text = File.ReadAllText($@"{pathname}", Encoding.UTF8);

I have several txt files with different conditioning. Because here does not show special characters, because of different encoding.

Before running the File.ReadAllText line, how do I get the file type?

Example: ANSI, UNICODE, UTF-8, ETC ...

Something like this:

if (pathname == Encoding.ASCII)
{
    string text = File.ReadAllText($@"{pathname}", Encoding.ASCII);
}
else if (pathname == Encoding.UTF8)
{
    string text = File.ReadAllText($@"{pathname}", Encoding.UTF8);
}