How to find out a language that used to build a website?

No one can say this programming language is better than other one , or worse but here is the list of top ten best and most popular programming languages ....
1.
Java : Its high performance extends its usability to more of the developers out there.
Its the only language which most dynamic , threaded and interpreted.
Its one feature that is automatic memory management leads it to far high compared to other programming languages.
2.
C# : hich is multi-paradigm encompassing the features like imperative , functional , object oriented , structured and component oriented.
It is mostly influenced by Java.
Its secure 3.
C++ 4.
JavaScript (along with CSS and HTML) 5.
Visual Basic 6.
PHP 7.
Objective - C 8.
Perl 9.
Python 10.
Ruby

Gmail is written in Java.
Bayt is written in TCL.
Wikipedia is written in PHP.
Many parts of google.com are written in Python.
Slashdot is written in Perl.
Twitter is written in Ruby.
Stackoverflow is written in C#.
The list goes on...
As you can see, there is no best language, a language is only what you make of it.

In general most IT folks don't care to hide the programming language, so you may check the site response header, some of them states the framework used to generate the page, for example you may get Django, so you can tell the language is python and so on.
Other sites have the files extension visible, you can easily check this, where you may find links ended with .php, .asp,.tcl....etc.
Another way is to check the page source, you can find a clue sometimes there, for example the "Post back" for forms submissions is a .Net thing.
Focus on Java-scripts, many modern frameworks provide javascripts and usually the functions names are unique to that framework.
But if the website owner didn't announce it publicly , all methods could be inaccurate, where it's easy to create a website over python and have all urls ended up with .php.

Php is good for website because its processing is fast than any other language on internet...

You may know the scripting language if the developer don’t hide the page extinction.
e.g.
ASPX or .PHP or .JSP and so on.
Know related to your question how can you know the programming language behind these scripting languages .For .Net you can’t do that directly because in .net I may have 5 group work in one project and each group programming in different programming language e.g C# and VB AND C++ .At the end all of them will end with .ASPX , so you can’t now the language behind the script .
For .JSP the programming language is Java and so on.
Most company and what we are recommended to hid the extension of the technology you are using for many reasons.
Security and Maintenance are the major reasons.