You can typically look within the database and file system to get this information without even logging into the site.
Check out this post and this post for more details.
Also, with some experience you can check the login page and get an idea if they are on a new or old version. If a site is using the free license, you can view the source code and see something like this in the header:
<meta name="generator" content="Kentico 8.0 (build 8.0.0) FREE LICENSE" />