[Aetae]

Експеримент в консольке FF:

function isNativeDOMObject(obj){
    if(obj.prototype) obj = obj.prototype;
    if(obj.constructor) obj = obj.constructor; 
    try{
        Function.prototype.toString.call(obj) 
    }catch(e){
        return true
    }
    return false
}
alert([
    'document.body',  isNativeDOMObject(document.body),
    'document',  isNativeDOMObject(document),
    'CSSStyleDeclaration',  isNativeDOMObject(CSSStyleDeclaration),
    'document.body.style',  isNativeDOMObject(document.body.style),
    'NaN',  isNativeDOMObject(NaN),
    'new String("abc")',  isNativeDOMObject(new String("abc")),
    'new XMLHttpRequest()',  isNativeDOMObject(new XMLHttpRequest()),
    'XMLHttpRequest',  isNativeDOMObject(XMLHttpRequest),
    'URL',  isNativeDOMObject(URL)

].join('\n'))

На верность результата не претендую, просто рандомно пойманная закономерность.)