foo.js

var EventEmitter = require('events').EventEmitter,
	callback = (function () {
		var called = 0,
			spy = function () { called += 1; };

		spy.callCount = function () {
			return called;
		};
	    
		return spy;
	}());


var emitter = new EventEmitter();

emitter.on('name', callback);
emitter.once('name', callback);

emitter.emit('name');

console.log(callback.callCount());