How can I know which version of Drupal is installed in my server?
Open project folder. Look for CHANGELOG.txt and open it. Here you can find version.
Log into Drupal admin interface. Then go to "Administer -> Available Updates". And you'll be able to see what drupal version your are using.
or you can go to the file /modules/system/system.info and you will see something like version = "6.2"
Indeed, looking at any .info file on your Drupal instance in any theme or module folder (inside /sites/all) may be easiest/quickest for you as opposed to adding PHP code though both are quite easy.
If for any reason you don't have FTP/SSH access to your Drupal server, there are other ways like viewing page source in the browser (if you know what look for) or a simple browser pluggin such as 'Drupal for Firebug' or similar utlity:
https://addons.mozilla.org/en-US/firefox/addon/drupal-for-firebug/
The really easy way is: go to your site, and into the Browser go to: view source.
Generally you can find:
<meta name="generator" content="Drupal 7 (http://drupal.org)" />
Or just press: Ctrl/Cmd "F": And find the word Drupal.
You will find the Drupal version in the code without check anything in the admin.
To easily check Drupal version, go to www.yourwebsite.com/CHANGELOG.txt
CHANGELOG.txt
will show you the version and change log updates to the Drupal build version.