With the SharePoint 2013 Public Update now available it can be useful to understand which build of SharePoint your site is running on . Here is a simple way to figure that out:
Type http://yoursitename/_vti_pvt/service.cnf (note the underscores before and after vti) into your favorite browser.
This command will return something that looks like this:
The last 4 digits of the extenderversion are your build number. The first 2 digits indicate the version (15 for 2013 and 14 for 2010)