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How can I tell what a website was written in?
Is there a way to find out what are the web technologies used to build certain websites ? For example I like this website : http://www.grouprecipes.com How can I know what technologies he used without contacting him ?
If you want to know what's on the backend, the short answer is that you cannot. You might be able to guess if they show script file names, but that isn't as common these days (REST). You are stuck with viewing source and looking for .jsp, .cfm, .php, etc. Client side, you can, of course, view source and see what javascript (jquery, etc) they are using.
I came over a Safari plugin earlier this week, that gathered some information about websites by fetching headers and mark-up, amongst other elements. Their homepage is http://builtwith.com/. I'm not sure how accurate it is, but it seems to produce good results. You can go directly to the results for grouprecipies.com by navigating to http://builtwith.com/grouprecipes.com.