Answer:
.com, .org, .gov, etc.
(I don't think the above is right)
This is a very broad question, I assume you are talking about the technology behind it. the best way is to understand whats out there and "view source" and see if you can recognize certain code patterns.
To tell what programming language is used for dynamic sites look for file extensions in the url like ".php" or ".aspx" of ".jsp"