How can you tell what language has been used to program a website?

Answer 1

Usually, you can't: the only visible thing is the final HTML/Java that reaches your computer, and that you can see with "View Source".
All other code that run on the server is invisible to you.
I.E. a code in PhP will access MySQL database, read data, format it then sends a page written in HTML to your machine. You can only see the HTML.
On the other hand, any complex language running on the server will have many calls to other sub-programs to which you still do not have access.
As said above, you can only guess with the extention (.php, .asp, .js etc...)
Addition: Look at the code: if it is full of  's and extremely badly readable (tabs, CRLFs etc) and contains multiple useless repeats such as text...
It has been developed using a front end for non-programmers, such as dreamweaver or similar.
These write crap code impossible to maintain.
If you see these, the the web-designer is just an amateur...
A Pro uses just a plain text editor, easier to use, clearer, but he KNOWS what he is doing!

Answer 2

You usually can't. As the above answer says, some web pages are identifiable because they end in .asp, .aspx (for ASP.NET) or .php. But some people regard this as a security risk - you should allow visitors to see as little information about your server setup as possible, to make it more difficult to hack. So many websites (mine included, and Yahoo) don't show what language has been used.

However, most of the time it is one of the above three, or possibly Python or Ruby.

Answer 3

For some website we can easily know by seeing
its extension on the address bar
such as .php means it is based on Php
.asp mean Asp
.jsp is Java

Answer 4

Look at the code (right click, View Source).