function isArray(arr) {
  return Object.prototype.toString.call(arr) === '[object Array]';
}
Source: https://github.com/timoxley/to-array

Convert an array-like object into an `Array`.
If `collection` is already an `Array`, then will return a clone of `collection`.

@param {Array | Mixed} collection An `Array` or array-like object to convert e.g. `arguments` or `NodeList`
@return {Array} Naive conversion of `collection` to a new `Array`.
@api public