testingexample.js

MyClass = {

  makeIOCall: function(url) {
    Y.io(url, {
      on: {
        success: this.successHandler
      }
    }
  }

  successHandler: function(id, o, args) {
    // do something with these params
    return Y.JSON.parse(o.responseText);
  }

}

// in your test
// using mocha syntax (convert to whatever engine you are using)
it('can properly parse server response data', function() {
  var data = myClass.successHandler('foo', { server: 'data'});
  assert(data.server, 'data', 'Did not parse json properly');
});