A nifty lil' snippet to wrap around .push() to observe what's being added to dataLayer.

dataLayer-observer.js

console.log('dataLayer before injection', window.dataLayer);
(function(){
  // Safeguard for sites that don't implement dataLayer or that haven't initialized it yet.
  window.dataLayer = window.dataLayer || [];
  // Keep a reference to the method we're overwriting
  var existing_func = window.dataLayer.push;
  window.dataLayer.push = function(){
    var args = arguments;
    Object.keys(args).forEach(function(key){

      // Here is where we should look for args[key] entries that match our criteria.
      // For now, we just print out whatever is being added.
      console.log('Pushing to dataLayer', args[key]);

    });
    // Now call the original method
    return existing_func.apply(this, args);
  };
})();
console.log('dataLayer after injection', window.dataLayer);

For reference, this is how GTM.js does it: