gistfile1.js

function methodIsDefined(fn, obj) {
  var ns = fn.split('.');
  fn = ns.pop();
  do {
    if (!ns[0]) {
      return typeof obj[fn] === 'function';
    }
  } while(obj = obj[ns.shift()]);
  return false;
}

// Object traversal method detection:

methodIsDefined('alert', window); // true 
methodIsDefined('window.alert', window); // true
methodIsDefined('alert.apply', window); // true
methodIsDefined('alert', {}); // false 
methodIsDefined('foo', window); // false
methodIsDefined('window.foo', window); // false
methodIsDefined('foo.bar.doesNotExist', {}); // false

@millermedeiros it does! Just not the way that you want :P