You can often tell about front-end technology. Sometimes it is obvious: if URL contains pages ending with .jsp, almost sure some J2EE server is involved (Tomcat, JBoss, Weblogic, ...). The ending ".asp" is usually old-style Microsoft ASP, .aspx is ASP.NET, .php is obviously PHP.
With ending like .do, .html it can be anything (quite often Java).
Often you can tell more by opening the source and reading on and/or looking into HTTP headers returned.
Where the things are really interesting is the middle ware and backend - is something like NoSQL database used or is it traditional relational database ? This is very hard to tell.
Best bet is either reading the Web site blog (if there is one) or trying to approach from other side: pick a technology (e.g. Cassandra) and read on their website in which projects is being used.