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Kinley, if you look at the HTML source, a drupal site will have a stylesheet in a location like
/sites/[sitename]/themes/[themename]
[sitename] will be either default, or some name that is based on the domain.
[themename] could be anything.
Go to some of the sites they list there and look for the common footprint.
At it's most obvious, some sites have
<meta name="Generator" content="Joomla! - Copyright (C) 2005 - 2006 Open Source Matters. All rights reserved." />
Or the equivalent. More recent ones include the version number (1.5 on I think).
Also, you'll typically, but not always, find stylesheets in
/templates/[template name]/css/template_css.css
but of course that may be the case on any site that uses some sort of template, but it's a good indicator.
Also, a little "Powered by Joomla!" notice at the bottom is a good sign.
Interestingly, the top-ranked Joomla showcase site is running... Wordpress!
check the file structures
This is sort of like fargo's suggestion. I don't think you do have the answer though. The main joomla site (joomla.org) for example, does not have its stylesheet in /templates/[templatename]/css. They're serving that off a CDN, so that address is
http://cdn.joomla.org/css/template.css
In this case the domain gives it away, but this does not match the default template install.
Outside of the template/theme, no other part of the file structure is exposed to the public, so knowing the file structure doesn't tell you much.