In Douglas Crockford's book "JavaScript:The Good Parts". He proposed one method to check whether an object is array or not.

var is_array = function (value) {

return value &&
typeof value === 'object' &&
typeof value.length === 'number' &&
typeof value.splice === 'function' &&
!(value.propertyIsEnumerable('length'));

};

First, we ask if the value is truthy. We do this to reject null and other falsy values.Second, we ask if the typeof value is 'object'. This will be true for objects, arrays,and (weirdly) null. Third, we ask if the value has a length property that is a number.This will always be true for arrays, but usually not for objects. Fourth, we ask if thevalue contains a splice method. This again will be true for all arrays. Finally, we ask if the length property is enumerable (will length be produced by a for in loop?)That will be false for all arrays.

We can use Object.prototype.toString.call(obj)=='[object Array]' to check whether an object is an array. It works even there are many frames in one webpage.

For example:

var arr=[];
alert(Object.prototype.toString.call(arr)=='[object Array]'); //Returns true

var arr=null;
alert(Object.prototype.toString.call(arr)=='[object Array]'); //Returns false

In ECMAScript 5, we can find the Array.isArray() method. It wil work as expected.

if(Array.isArray(arr)){

//arr is an array

}