How to detect native browser/engine objects in javascript

I need a function to return true if the given variable name is already defined by javascript engine or browser.

For example

isNative('window') //true;
isNative('Math') //true;
isNative('myVar') //false
isNative('navigator') //true
isNative('document') //true

Ok I found the solution.

function isNative(variableName) {
    if (window['__emptyIframeRef__'] === undefined) {            
        window['__emptyIframeRef__'] = document.createElement('iframe');
        window['__emptyIframeRef__'].setAttribute('style', 'display:none');
        document.getElementsByTagName('body')[0].appendChild(window['__emptyIframeRef__']);
    }
    return window['__emptyIframeRef__'].contentWindow[variableName] !== undefined;
}

Additional code that can be added to the top to guard against edge condition of code running before document.body is available:

function isNative(variableName) {
    if(o===null||o===undefined)return true
    if(!document.body){
        if(o instanceof String) o = window[o]
        if(o===undefined) return false
        if(o.constructor.toString().indexOf('[native code]')>-1) return true
        return false
    }
    ...
}