WordPress at a glance
function is not described

is_subdomain_install() WP 3.0.0

Whether a subdomain configuration is enabled.

No Hooks.

Return

true/false. True if subdomain configuration is enabled, false otherwise.

Usage

is_subdomain_install();

Code of is subdomain install: wp-includes/ms-load.php VER 5.0.3

<?php
function is_subdomain_install() {
	if ( defined('SUBDOMAIN_INSTALL') )
		return SUBDOMAIN_INSTALL;

	return ( defined( 'VHOST' ) && VHOST == 'yes' );
}