Interval.js

window.myInterval = setInterval(function() { 
  
  if ($("some-selector").length > 0) { //looks for a selector
  
    /*
    do something here
    or add/or in a custom event goal like below:
    */
    window['optimizely'] = window['optimizely'] || [];
      $("someselector").bind("mousedown", function() {window.optimizely.push(["trackEvent", "eventName"]); 
    });
    
    clearInterval(window.myInterval); //after it's found, clear the interval
  }
}, 50) //in milliseconds;