Source

File: wp-includes/load.php

function wp_is_xml_request() {
	$accepted = array(
		'text/xml',
		'application/rss+xml',
		'application/atom+xml',
		'application/rdf+xml',
		'text/xml+oembed',
		'application/xml+oembed',
	);

	if ( isset( $_SERVER['HTTP_ACCEPT'] ) ) {
		foreach ( $accepted as $type ) {
			if ( false !== strpos( $_SERVER['HTTP_ACCEPT'], $type ) ) {
				return true;
			}
		}
	}

	if ( isset( $_SERVER['CONTENT_TYPE'] ) && in_array( $_SERVER['CONTENT_TYPE'], $accepted, true ) ) {
		return true;
	}

	return false;
}