You are correct in the general approach, i.e. to search for error messages the system gives. In the particular case, being web sharing managed by the apache server the obvious place to examine was in effect its directory /private/etc/apache2 and its configuration file httpd.conf
But beware of generalizing our personal experience: in my particular case, I had to comment both fastcgi and php5 module lines in httpd.conf in order to have web sharing correctly functioning ;-)
This approach works fine also in other situations: the rule-of-thumb is to revert to the ancient Unix times and to work with command line (but this was annoying, and was the reason why I chose Apple instead of Linux :-( ).
By the way, I'm VERY disappointed with Apple: they sell a product (Lion Server) far away from their last and best real server product (Leopard and Snow Leopard Server). Letting alone the tragic decision of discontinuing the glorious server hardware (Xserve family)... I think Apple is at the limit of a fraud to its customers!