Has this constructor been prototyped?

This is a quick post about libraries that uses native constructor in an obtrusive way.
Using a Function prototype, that sounds like a non-sense, it is possible to know if a constructor has some property, or method, defined in its prototype.

Function.prototype.prototyped = function(){
    for(var i in new this)
        return true;
    return false;
};

Some test example?

alert(Array.prototyped()); // false

Object.prototype.each = function(){};
alert(Array.prototyped()); // true

delete Object.prototype.each;
alert(Array.prototyped()); // false

Array.prototype.each = function(){};
alert(Array.prototyped()); // true
alert(Object.prototyped()); // false

Update
Has Kangas spotted, there is no reason to create a new instance.
We could loop directly the prototype object.
But what's up if we have injected privileged methods in the constructor?

Array = function(Array){
    var prototype = Array.prototype;
    return  function(){
        arguments = prototype.slice.call(arguments, 0);
        arguments.each = function(){
            prototype.forEach.apply(this, arguments);
        };
        return  arguments;
    };
}(Array);

var a = new Array(1, 2, 3);
a.each(function(value){
    alert(value)
});

for(i in Array.prototype)
    alert(i); // nothing

Above example is the reason I chose to create an instance ... but hey, why on heart someone should wrap a native constructor? :P