How to tell if a site is using Drupal 7
Some obvious giveaways to the site being drupal 7:


RDF in the &lt;head&gt; section in the HTML
Meta tag of &lt;meta name="Generator" content="Drupal 7 (http://drupal.org)" /&gt;
Executing jQuery.fn.jquery; reveals the jQuery version to be 1.4.4 or 1.5.2 (with jQuery update)
Only a single &lt;meta http-equiv="Content-Type" content="text/html; charset=utf-8" /&gt; - this is a famous drupal 6 bug (it has two in the &lt;head&gt; section
HTTP response header of X-Generator: Drupal 7 (http://drupal.org)


Other options include using 'sniffers', my favourite being 'Whatweb' - https://github.com/urbanadventurer/WhatWeb. Whatweb is a (ruby) command line tool that can sniff not only the fact that the site is using drupal, but often the major and minor version as well