http://www.mydrupal.ca/2010-06-23/what-version-drupal-your-site-using has a helpful check list to go through when you need to determine the drupal version with missing Changelog files:
1. First thing to do is to read the CHANGELOG.txt file. It’s located in the root directory of your Drupal website. All versions of Drupal from 4.5-present have this file. Check the top entry in this file. It will have the Drupal version number. Unfortunately for our purposes today, many developers delete this file on live sites because it is perceived as a security risk.
2. Second thing to do is to check the VERSION constant in modules/system.module. This was added in Drupal 5.0.
3. If you can’t find the VERSION constant in modules/system.module then your version number is most likely 4.7 or earlier. In Drupal 4.7, the site ‘offline for maintenance’ feature was added.
4. In Drupal 4.6, a site-wide contact form (modules/contact.module) and search block were added.
5. In Drupal 4.5, all GIF’s in the default themes were replaced with PNG’s. If you can’t find the modules/contact.module, then your site is most likely Drupal 4.5.
6. You can actually download the 4.x versions on Drupal.org. Then you can compare the files in more detail. This article covers the major releasees and not the minor releases. For example upgrading from 4.51-4.52, is called a minor release.