How know what theme/theme name a Drupal site is using? Like in Wordpress, we can know what theme name a site is using by look at the page source and it will show something like 

wp-content/theme/theme-name


Well, how we can know what theme a Drupal site is using? I try to look at the page source of a Drupal site but i cannot figure out what theme or theme name the site is using. 

Is there any trick to unlock this?
You can get it from source of any page. Have a look for the javascript array at the top/bottom of the page that begins with:

jQuery.extend(Drupal.settings, {...


There's a setting in that array called "theme" which contains the name of the theme being used for that page.