How to find out what programming language a website is built in?

There's no way to be 100% sure if you don't have access to the server, so it's about guessing. Here are some clues:

• File extensions: login.php is most likely a PHP script.
• HTTP headers: they may leak some information about the language which is running on the server, and some additional details like the version: X-Powered-By: PHP/7.0.0 means that the page was rendered by PHP.
• HTTP Parameter Pollution: if you managed to guess which server is running, you can refine the guess.
• Language limits: maximum post data, maximum number variable in GET and POST data, etc. It may be useful if the webmaster kept the default values.
• Specific input: for example, PHP had some easter eggs.
• Errors: triggering errors may also leak the language. Warning: Division by zero in /var/www/html/index.php on line 3 is PHP, for example.
• File uploads: libraries may add metadata if the file is being modified server-side. For example, most sites resize users' avatars, and checking for EXIF data will leak CREATOR: gd-jpeg v1.0 (using IJG JPEG v90), default quality, which may help to guess which language is used.
• Default filenames: Check if / and /index.php are the same page.
• Exploits: reading a backup file, or executing arbitrary code on the server.
• Open source: the website may have been open-sourced and is available somewhere on Internet.
• About page: the webmaster may have thanked the language community in a "FAQ" or "About" page.
• Jobs page: the development team may be recruiting, and they may have detailed the technologies they're using.
• Social Engineering: ask the webmaster!
• Public profiles: if you know who is working on the website (check LinkedIn and /humans.txt), you can check their public repos or their skills on online profiles (GitHub, LinkedIn, Twitter, ...).

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You may also want to know if the website is built with a framework or a CMS, since this will give information about the language used:

• URLs: directories and pages are specific to certain CMS. For example, if some resources are located in the /wp-content/ directory, it means that WordPress have been used.
• Session cookies: name and format.
• CSRF tokens: name and format.
• Rendered HTML: for example: meta tags order, comments.

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Note that all information coming from the server may be altered to trick you. You should always try to use multiple sources to validate your guess.

For guessing the programming language, you can follow the three steps approach detailed below:

STEP 1 - Search evidences on the site itself

Manually...

• Search on a site page at the bottom for phrases like:

  -> "Powered by XXX"
  -> "Proudly Powered by XXX"
  -> "Running on XXX"
  -> ...
  

• Search on the site if it will attend any conference where they could talk about the website from a technical point of view

...or with the help of a tool

• Read the HTML code downloaded by your browser

• Fire up the Network Tab in developer toolbar and study the exchanges made between the browser and the server.

• Search for some known hidden page:

  wget -head http://the-site.com/private/admin

  If you get 200, the site may be running on a plublicly (free, paid etc) available software.

STEP 2 - Search evidences on the web

Ask search engines for front-end errors

You can look for some errors produced by the website.

• Some keywords to type in a search engine:

  • Error 500 site:the-site.com
  • Exception site:the-site.com
  • ...
  • <what ever> site:the-site.com
    => You can simply replace "<what ever>" with some known error message produced by the various web technologies.

Ask search engines for back-end errors

You can even guess the technologies used in the backend:

• ORA-12170 site:the-site.com
  => If you find something, the site may be using Oracle in its backend part.

Ask search engines for website competitors

• Find what technology is popular in the website industry

• Find what technology competitors are using

• Find comparisons of the site with other competitors.
  Those comparisons may talk about technologies in use

Technology survey sites

Those sites can provide great info to the the site you target. They may have already done some part of the job for you.

• http://w3techs.com/sites
  => Enter the url of the site you're targetting and see what technologies (client or server side) have been detected.
  Note that the site must be in the top 1M Alexa ranking.

• http://stackshare.io/search/q=<keyword>
  => <keyword> can be anything company name, website name, etc

STEP 3 - Analyze your results

The evidences you have found in step 1 may be wrong because the site owner can alter them. Try to find contradictions between those evidences. Eliminate contradictional evidences.

Merge the evidences in step 2 between the various sources and yours. Again eliminate contradictional evidences.

Resume all your findings in a table like the one below.

+-------------+-----------+------------------+    ...   +----------+-------+--------+
| EVIDENCES   |  ON SITE  |  Search Engine 1              SOURCE n   SCORE   PCT (%)
+-------------+------------------------------+    ...   +----------+-------+--------+
|    PHP 7    |     X     |       X          |                X    |   3   |  300/n
+-------------+------------------------------+    ...   +----------+-------+--------+
|  Wordpress  |           |       X          |                X    |   2   |  200/n
+-------------+------------------------------+    ...   +----------+-------+--------+
     ...
+-------------+------------------------------+    ...   +----------+-------+--------+
|  EVIDENCE m |           |                  |                     |       | (100*SCORE)/n
+-------------+------------------------------+    ...   +----------+-------+--------+

Finally, you will be able to say "I'm confident at XX% that this site runs on YY (EVIDENCE i)".

It's simple. Add Wapplyzer extension available for Chrome as well as Firefox.

It tells about programming language, server, analytics tool or about CMS & Frameworks on which website is built.

Give it a try, you will love it.