includes/class-freemius.php
function is_addon_installed( $id_or_slug ) {
            $this->_logger->entrance();

            $addon_id = self::get_module_id( $id_or_slug );

            return file_exists( fs_normalize_path( WP_PLUGIN_DIR . '/' . $this->get_addon_basename( $addon_id ) ) );
        }
function is_addon_installed( $id_or_slug ) {
            $this->_logger->entrance();

            $addon_id = self::get_module_id( $id_or_slug );

            return file_exists( fs_normalize_path( WP_PLUGIN_DIR . '/' . $this->get_addon_basename( $addon_id ) ) );
        }
function is_addon_installed ( $ id_or_slug ) { $ this -> _logger -> entrance ( ) ; $ addon_id = self :: get_module_id ( $ id_or_slug ) ; return file_exists ( fs_normalize_path ( WP_PLUGIN_DIR . '/' . $ this -> get_addon_basename ( $ addon_id ) ) ) ; }
Determines if add-on installed.

NOTE: This is a heuristic and only works if the folder/file named as the slug.

@author Vova Feldman (@svovaf)
@since  1.0.6

@param  string|number $id_or_slug

@return bool
Determines if add - on installed .