How to Determine What Technology a Website Is Built On

How do you determine what technology a website is built on?

There are all sorts of things you can look out for, but it doesn't really give you any certainty as to the technology behind a site. In general, information like that is something people will want to hide, as the more information that is exposed the easier it might be for malicious parties to identify security vulnerabilities or denial of service holes.

If I was interested I'd probably look, in no particular order, at:

urls, and file extensions.
HTTP response headers
Source code for comments, or standard JS libraries

Incidentally, the tools mentioned in other answers are only looking at some of the above properties of the site for you, albeit automatically, and slightly quicker. :)

How to determine what technology google used to build the website?

There is this amazing website that provides you all the info regarding technologies used by particular websites. Try it out http://builtwith.com/

Udemy and Upwork has built on which technology? (technology used for website and for native app (Android & iOS))

React and React Native could be good options if you like to keep the development process similar through web- and app development. My second recommendation would be to use Apache Cordova/Adobe Phonegap.

Maybe BuiltWith could be a useful tool to check what technology is used to build a specific website.

How to tell if a web application is using ReactJs

try the below snippet, thanks for the examples for each site listed by rambabusaravanan. See the below link

if(!!window.React ||
   !!document.querySelector('[data-reactroot], [data-reactid]'))
  console.log('React.js');

if(!!window.angular ||
   !!document.querySelector('.ng-binding, [ng-app], [data-ng-app], [ng-controller], [data-ng-controller], [ng-repeat], [data-ng-repeat]') ||
   !!document.querySelector('script[src*="angular.js"], script[src*="angular.min.js"]'))
  console.log('Angular.js');

if(!!window.Backbone) console.log('Backbone.js');
if(!!window.Ember) console.log('Ember.js');
if(!!window.Vue) console.log('Vue.js');
if(!!window.Meteor) console.log('Meteor.js');
if(!!window.Zepto) console.log('Zepto.js');
if(!!window.jQuery) console.log('jQuery.js');

you can find additional info here link

how can I get to know which server side programming is used by the website?

Each server side programming language has their own quarks which make them distinct. w3af has a number of "Discovery" plugins for gaining information about a remote server. A good example is the w3af plugin PHPEggs.

  This plugin tries to find the documented easter eggs that exist in PHP and identify
    the remote PHP version using the easter egg content. The easter eggs that this plugin
    verifies are:

        - http://php.net/?=PHPB8B5F2A0-3C92-11d3-A3A9-4C7B08C10000 ( PHP Credits )
        - http://php.net/?=PHPE9568F34-D428-11d2-A769-00AA001ACF42 ( PHP Logo )
        - http://php.net/?=PHPE9568F35-D428-11d2-A769-00AA001ACF42 ( Zend Logo )
        - http://php.net/?=PHPE9568F36-D428-11d2-A769-00AA001ACF42 ( PHP Logo 2 )