function isArray(o) {
  return Object.prototype.toString.call(o) === '[object Array]'; 
}

The Miller Device

The JavaScript language currently does not provide a good way to distinguish between objects and arrays. The typeof operator is broken: It identifies arrays as objects. Comparing a value's constructor property doesn't work because arrays created in a different frame will have a different constructor. There are do-it-yourself tests for arrayness, but they are complicated and unreliable.

Mark Miller of The Google, by closely reading the ECMAScript standard, has discovered a simpler, more reliable test.

Object.prototype.toString.apply(value) === '[object Array]'