<!-- some code before -->
<div id="bottomAd" style="font-size: 2px;">&nbsp;</div>
<!-- some code after -->
 
$(document).ready( function() {
  window.setTimeout( function() {
    var bottomad = $('#bottomAd');
    if (bottomad.length == 1) { // может быть лучше так? if (typeof bottomad !== "undefined") 
      if (bottomad.height() == 0) {
        // adblocker active
      } else {
        // no adblocker
      }
    }      
  }, 1);
}