spy.js

var util = require('util');
var EventEmitter = require('events').EventEmitter;

function Spy(f, ctx) {
  var ee = new EventEmitter();

  spy.called = false;
  Object.keys(EventEmitter.prototype).forEach(function(p) {
    spy[p] = transfer;

    function transfer() {
      var args = Array.prototype.slice.call(arguments);
      return ee[p].apply(ee, args);
    }
  });

  return spy;

  function spy() {
    var args = Array.prototype.slice.call(arguments);
    spy.called = true;
    spy.emit('called', args);
    var ret = f.apply(ctx || null, args);
    spy.emit('returned', args, ret);
    return ret;
  }
}

// Usage

function theCallback() {
}

var spy = Spy(theCallback);

spy.on('called', function() {
  console.log('Gotcha!', spy.called);
});

spy();