how to know the platform that a particular website is running on ?

StatCounter will only show you Browser type and O/S
Google Analytics will show you Browse type, O/S, network location, connection speeds, Flash versions

You might have to go with javascript for this. Here are a few code sample repositories:

http://www.dynamicdrive.com/
http://getelementbyid.com/scripts/index.aspx
http://www.brainjar.com/
http://www.scriptasylum.com/
http://www.webreference.com/js/
http://webdeveloper.earthweb.com/webjs/

You could check out Nikto (http://www.cirt.net/code/nikto.shtml).  NMap (http://insecure.org/nmap/) is a great tool as well, but it has a bit of a learning curve

Also you can add Fiddler to IE or LiveHttpHeaders to Firefox.  Run the tool, go to a site and whent eh page is loaded you will find info like this in the tools (exampe from HttpLive Headers):

--------------------------------------------------------------------------
http://livehttpheaders.mozdev.org/installation.html#

GET /installation.html HTTP/1.1
Host: livehttpheaders.mozdev.org
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.6) Gecko/20070725 Firefox/2.0.0.6
Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Cache-Control: max-age=0

HTTP/1.x 200 OK
Date: Wed, 08 Aug 2007 02:09:08 GMT
Server: Apache
Vary: Host
X-Powered-By: PHP/4.4.7
Keep-Alive: timeout=2, max=15000
Connection: Keep-Alive
Content-Type: text/html
Content-Encoding: gzip
Content-Length: 8346