Frankly speaking, when you are browsing the web page in browser, there is only one way you can possibly understand which server-side language have been used to develop the page. That is, the URL extension. Like, this page (This CodeProject web page have been written in Asp.net, because, I can see it has an extension .aspx).
But, these days, extension less URLs are popular, and, many site don't have any URL extension. In such cases, you can view source of the web page and look for any URL extension. However, if you can't find any extension, I don't see any way of telling which language is used to develop the page.