can any one tell me how can we check website platform, either it is wordpress, drupal, joomla etc etc.
Most of those sites have demos where you can log in to the admin and frontend sections of an example site. The besy way I think is to just download each one and run an install. It depends what you need it for in the end. Wordpress is the easiest to configure and theme, Joomla is fairly complicated and I've not used Drupal, but assume it is similar to Joomla. If you haven't experienced a CMS before, it may be best to start small and work your way up, they all work fairly similar.
If you want to check what CMS a site is running, just look at the source. Look at the css file. And also look at the template and plugins directory. It's usually pretty obvious.
I believe if you are using free template of Wordpress, Jooml, or any other CMS it gives link in footer with author name who changed the theme, many time in view source of HTML you can look for special Meta tags of such CMS. above all I agree with "Kerosene" to check the CMS used for any sites. Thanks!
In the source, look for the generator meta tag.. it will be like. <meta name="generator" content="Wordpress"/> or <meta name="generator" content="Joomla"/> or <meta name="generator" content="Blogger"/> etc;