How can I detect the development language / platform of a web application?

Question by MC: How can I detect the development language / platform of a web application?
Is there a way to understand the the development platform (like asp.net, php etc.) of a web application / web site by just looking the source from the web browser?

While surfing the net I usually got curious about the platform the web page uses. Usually it is easy to understand it by checking the extensions. However, some web sites hide the extensions or they can fake the extensions.

Best answer:

Answer by Steve
There’s no guaranteed way to know for sure, because these are server side technologies and you’re trying to check from the client side. However, there are sometimes clues:

A) The URL: As you mention, you can sometimes tell from the URL, but they can hide the extension or even rewrite it to something else.

B) The Source Code: You can sometime work out which server side technology is being used by looking at the source code. There’s no single thing to look for in here, so my best recommendation is to search for .asp or .php in the code. For example if you look at the source of this page right here on Yahoo! Answers, you’ll see things like: