Function to Determine if Multisite is Active and Enabled

ms-detection.php

<?php
/**
 * Checks if the plugin is on a multisite install.
 *
 * @param bool   $network_admin Check if in network admin.
 * @param bool   $network_activated Check if plugin is network activated.
 * @param string $plugin_file Path to the plugin file.
 *
 * @return true if multisite, false if not.
 */
function dlx_is_multisite( $network_admin = false, $network_activated = false, $plugin_file = __FILE__ ) {
	if ( ! function_exists( 'is_plugin_active_for_network' ) ) {
		require_once ABSPATH . '/wp-admin/includes/plugin.php';
	}
	if ( $network_admin ) {
		if ( is_network_admin() ) {
			if ( is_multisite() ) {
				if ( $network_activated ) {
					if ( is_plugin_active_for_network( plugin_basename( $plugin_file ) ) ) {
						return true;
					}
				} else {
					return true;
				}
			}
		} else {
			return false;
		}
	}
	if ( is_multisite() ) {
		if ( $network_activated ) {
			if ( is_plugin_active_for_network( plugin_basename( $plugin_file ) ) ) {
				return true;
			}
		} else {
			return true;
		}
	}
	return false;
}