Here is the simplest way to check if AdBlock is enabled on your site:

First of all you should create simple javascript file with following content:

var isAdsDisplayed = true;

Name it adsbygoogle.js and upload to your web server.

Then on the page where you want to check if AdBlock is enabled or not just add the following script:

<script src="/js/adsbygoogle.js"></script>
if(window.isAdsDisplayed === undefined ) {
  // AdBlock is enabled. Show message or track custom data here

You can use this method to show some message to users or track how many users are using AdBlock.