Is very easy right click ->view source code...and loook on meta tags
Or some time you cand look on style.css and it say what script is.
But is very important that some time to know the structure of that script,Like Joomla Wordpres ,Drupal ,Magento they have a special signature and you will know that script is used,
But some time some websites are CUSTOM MVC, and is hard to fiind that script.