input.xml

<?xml version="1.0" encoding="windows-1251"?>
<root>
    <!-- Save this file with windows-1251 encoding! -->
    <hello>Привет, товарищ!</hello>
</root>

xml-test.js

var libxml = require('libxmljs'), doc;
try {
    doc = libxml.parseXmlFile('input.xml');
} catch (e) {
    // output is:
    // Input is not proper UTF-8, indicate encoding !
    // Bytes: 0xCF 0xF0 0xE8 0xE2
    console.log(e.message);
}