I've been trying to find the charset of a norwegian (with a lot of ø, æ, å) txt file written on a Mac, i've found it in this way:

<?php
$text = "A strange string to pass, maybe with some ø, æ, å characters.";

foreach(mb_list_encodings() as $chr){
        echo mb_convert_encoding($text, 'UTF-8', $chr)." : ".$chr."<br>";    
 } 
?>

The line that looks good, gives you the encoding it was written in.

Hope can help someone