Checking if object is a DOM-element

Passing DOM elements to WebWorkers gets tricky since all references to the DOM are 'lost'. I need to check objects that are passed before the WebWorker's message is sent.

What is the fastest way to check if an instance of an object is a DOM-element OR/AND part of the DOM tree, OR has 'children' that contain any references to the DOM tree?

piece of the usage:

var a = new SharedWorker("bigdatahandler.js");   
a.postMessage(s);

s //<--should not be a DOM object