How to make sure that some site is developed using dotnetnuke??
I don't think there is a single guaranteed way to detect a DNN site by examining the HTTP headers or HTML output by the site.
URLs not a good indicator as many DNN sites re-configure or change the URL provider to adapt the URLs to their preferences.
A strong indicator is to look at where the .css files are loaded from. Most sites will have a /Portals/_default/Default.css file in each page, it's not an absolute but is very common.
In general any site using paths from Portals/_default/ or Portals/n/ where n is a number is likely a DNN site.
Most DNN pages that make use of javascript will include several files named dnn*.js . Again this isn't required in every page, but exists in most pages.
May be I'm late to answer this, but those who want to see the example sites developed using dotentnuke can go here:http://www.dotnetnuke.com/Products/NewtoDotNetNuke/DotNetNukeShowcaseSites/tabid/541/Default.aspx