The "Object.prototype.toString.call" solution is guaranteed to work according to ECMA-262, and will only return true for a real Array instance (not something masquerading as an Array):

Object.prototype.toString ( ) When the toString method is called, the following steps are taken:

1. If the this value is undefined, return "[object Undefined]".

2. If the this value is null, return "[object Null]".

3. Let O be the result of calling ToObject passing the this value as the argument.

4. Let class be the value of the [[Class]] internal property of O.

5. Return the String value that is the result of concatenating the three Strings "[object ", class, and "]".