Oh, it&#x27;s very easy:<p><pre><code>  &lt;img src=&quot;&#x2F;an_existing_image.jpg?&amp;ad_box_&quot; onerror=&quot;alert(&#x27;You are using adblock&#x27;)&quot;&gt;
</code></pre>
How it&#x27;s made:<p>1. Look into the most popular filter&#x27;s used by adblock extensions:
<a href="https:&#x2F;&#x2F;easylist-downloads.adblockplus.org&#x2F;easylist.txt" rel="nofollow">https:&#x2F;&#x2F;easylist-downloads.adblockplus.org&#x2F;easylist.txt</a><p>2. Choose for example a querystring rule from the beginning (from the &quot;General advert blocking filters&quot;)<p>3. Append it to the url of an existing image<p>4. Use the onerror event to detect if it wasn&#x27;t loaded