One technique hackers use to determine if you're using a website content management system is to simply search your site for commonly used back-end access addresses such as /admin or in the case of Joomla /administrator. You can hide your Joomla Administrator login page by using an extension called jSecure Authentication.
This simple extension requires you to add a word that you specify to the Administrator address. So if you choose "chocolate-egg" as your word, you would then access your Administrator by going to http://www.yoursite.com/administrator?chocolate-egg. Anyone trying to access your site using http://www.yoursite.com/administrator will be redirected to an error page or your home page.