Is there are a way (a library) with which a web page can detect a screen reader being used on it? This can be just for a reporting/analytics purpose.
PS: A Drupal 8 website.
No. It is not possible. Screen readers operate as an application on the computer - this would be similar to trying to find out if someone viewing your website also had their calculator open - it is a privacy restriction. Also many of these users may be using Voiceover or Talkback on their mobile devices and there is no way to detect that either.
Your website should instead strive to follow web standards and work equally for all users.
I'm also curious as to what your specific goal is in detecting this, as screen readers are only one part of the accessibility tools that many people use - and focusing on just the screen reader user will not make your site accessible.