There are multiple ways to find out which version of WordPress a website is currently using.

1. Via the WordPress backend
After you logged into the admin area you can see the current version displayed at the bottom right of each page.

2. Via the filesystem.
If you have access to the PHP files check wp-includes/version.php. The variable wp_version contains the currently running version of WordPress, e.g.

$wp_version = '3.3.1';

3. Via the web browser
If you cannot login to the admin area and don’t have access to the file system you might be able to see the current version in the HTML source code. If not disabled WordPress will by default show a meta tag with the current version information.

The RSS feed might also contain version information.