Just paste the code below in your theme file, wherever you’d like to check out if a plugin is activated or not. Update the code with the plugin directory and name on line 4.
<?php include_once( ABSPATH . 'wp-admin/includes/plugin.php' ); if(is_plugin_active('plugin-directory/plugin-file.php')) { //plugin is activated } ?>
Thanks to Jean galea for this brillant tip!