Check your CHANGELOG.txt
If you do not have admin access to your drupal site, you can quickly check the version via the CHANGELOG.txt, eg. domain.com/CHANGELOG.txt where domain.com is the url to your drupal site
The final release for Drupal 6 was 6.38 released in Feb 2016. i.e. no further release for Drupal 6. You are very much on your own unless you get 3rd party support from companies like https://www.mydropwizard.com/drupal-6-lts which also releases their code at https://github.com/d6lts
Upgrade to the LTS version as soon as you can as sites on earlier versions have been reported to have been hacked or have their database and files system deleted. However, do consider upgrading to Drupal 8 as there are several improvements in areas of security and performance.
Similarly, the latest release for Drupal 7 should be 7.69. Upgrade immediately if you are on earlier version as there were several security issues.
