Looking at the website or looking at website source code... How do you come to know on which platform is the site made? - Joomla- Wordpress-Drupal etc?
Wordpress is usually fairly easy to spot, somewhere in the source code (in the head section) you'll normally find a line as follows which loads the themes style sheet: <link type="text/css" rel="stylesheet" href="http://www.whatever.com/wp-content/themes/themename/style.css" /> the wp-content directory gives away the fact that this is powered by wordpress
I use wappalyizer for about a week, its nice as you can also see a lot of other tools running like jquery, tracking tools etc....
Website source code helps, if it doesn't then knowledge of files structure will always give the right answer.