How to determine if an object is an object literal in Javascript?

It sounds like you are looking for this:

function Foo() {}

var a = {};
var b = new Foo();

console.log(a.constructor == Object); // true
console.log(b.constructor == Object); // false

The constructor property on an object is a pointer to the function that is used to construct it. In the example above b.constructor == Foo. If the object was created using curly brackets (the array literal notation) or using new Object() then its constructor property will == Object.

Update: crescentfresh pointed out that $(document).constructor == Object rather than being equal to the jQuery constructor, so I did a little more digging. It seems that by using an object literal as the prototype of an object you render the constructor property almost worthless:

function Foo() {}
var obj = new Foo();
obj.constructor == Object; // false

but:

function Foo() {}
Foo.prototype = { objectLiteral: true };
var obj = new Foo();
obj.constructor == Object; // true

There is a very good explanation of this in another answer here, and a more involved explanation here.

I think the other answers are correct and there is not really a way to detect this.